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System-Level Condenser Evaporator (2P-MA)

Heat exchanger between two-phase fluid and moist air networks, with model based on performance data

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  • System-Level Condenser Evaporator (2P-MA) block

Description

The System-Level Condenser Evaporator (2P-MA) block models a heat exchanger between a two-phase fluid network and a moist air network.

The block can act as a condenser or as an evaporator in a refrigeration system, depending on the direction of heat transfer. The block model is based on performance data from the heat exchanger datasheet, rather than on the detailed geometry of the exchanger, and therefore lets you easily adjust the size and performance of the heat exchanger during design iterations, or model heat exchangers with uncommon geometries. You can also use this block to model heat exchangers with a certain level of performance at an early design stage, when detailed geometry data is not yet available.

You parameterize the block by the nominal operating condition. The heat exchanger is sized to match the specified performance at the nominal operating condition at steady state.

The Two-Phase Fluid 1 side approximates the liquid zone, mixture zone, and vapor zone based on the change in enthalpy along the flow path. The Moist Air 2 side models water vapor condensation based on convective water vapor mass transfer with the heat transfer surface. Condensed water is removed from the moist air flow.

This block is similar to the Condenser Evaporator (2P-MA) block but uses a different parameterization model. The table provides a comparison of the two blocks, to help you choose the right block for your application.

Condenser Evaporator (2P-MA)System-Level Condenser Evaporator (2P-MA)
Block parameters are based on the heat exchanger geometryBlock parameters are based on performance and operating conditions
Heat exchanger geometry may be limited by the available geometry parameter optionsModel is independent of the specific heat exchanger geometry
You can adjust the block for different performance requirements by tuning geometry parameters, such as fin sizes and tube lengthsYou can adjust the block for different performance requirements by directly specifying the desired heat and mass flow rates
Lets you select between parallel, counter, or cross flow configurationsDoes not provide a choice of flow arrangement options
Predictively accurate results over a wide range of operating conditions, subject to the applicability of the E-NTU equations and the heat transfer coefficient correlationsVery accurate results around the specified operating condition; accuracy may decrease far away from the specified operating conditions
Heat transfer calculations account for the variation of temperature along the flow path by using the E-NTU modelHeat transfer calculations approximate the variation of temperature along the flow path by dividing it into three segments
Accounts for different fluid properties and heat transfer coefficients for subcooled liquid, liquid-vapor mixture, and superheated vaporAccounts for different fluid properties and heat transfer coefficients for subcooled liquid, liquid-vapor mixture, and superheated vapor
Keeps track of variable zone length fractions for subcooled liquid, liquid-vapor mixture, and superheated vapor regions based on the geometryApproximates the effect of subcooled liquid, liquid-vapor mixture, and superheated vapor regions using weighting factors based on the difference in enthalpy between inlet and outlet
Accounts for water vapor condensation and the latent heat on the moist air flowAccounts for water vapor condensation and the latent heat on the moist air flow
Does not model the wall thermal mass; you can approximate the effect by connecting a pipe block with a thermal mass downstreamIncludes an option to model the wall thermal mass

Heat Transfer

The two-phase fluid flow and the moist air flow are each divided into three segments of equal size. Heat transfer between the fluids is calculated in each segment. For simplicity, the equation for one segment is shown here.

If the wall thermal mass is off, then the heat balance in the heat exchanger is

Qseg,2P+Qseg,MA=0,

where:

  • Qseg,2P is the heat flow rate from the wall (that is, the heat transfer surface) to the two-phase fluid in the segment.

  • Qseg,MA is the heat flow rate from the wall to the moist air in the segment.

If the wall thermal mass is on, then the heat balance in the heat exchanger is

Qseg,2P+Qseg,MA=MwallcpwallNdTseg,walldt,

where:

  • Mwall is the mass of the wall.

  • cpwall is the specific heat of the wall.

  • N = 3 is the number of segments.

  • Tseg,wall is the average wall temperature in the segment.

  • t is time.

The heat flow rate from the wall to the two-phase fluid in the segment is

Qseg,2P=UAseg,2P(Tseg,wallTseg,2P),

where:

  • UAseg,2P is the weighted-average heat transfer conductance for the two-phase fluid in the segment.

  • Tseg,2P is the weighted-average fluid temperature for the two-phase fluid in the segment.

The heat flow rate from the wall to the moist air in the segment is

Qseg,MA=UAseg,MAc¯pseg,MA(h¯seg,wallh¯seg,MA)+m˙w,seg,condhl,wall,

where:

  • UAseg,MA is the heat transfer conductance for the moist air in the segment.

  • c¯pseg,MA is the moist air mixture specific heat per unit mass of dry air and trace gas in the segment.

  • h¯seg,wall is the moist air mixture enthalpy per unit mass of dry air and trace gas at the average wall segment temperature.

  • h¯seg,MA is the moist air mixture enthalpy per unit mass of dry air and trace gas in the segment.

  • m˙w,seg,cond is the rate of water vapor condensation on the wall surface.

  • hl,wall is the specific enthalpy of liquid water at the average wall segment temperature.

Using mixture enthalpy in this equation accounts for both differences in temperature and differences in moisture due to condensation [3].

Note

For the moist air quantities, the bar above the symbols indicates that they are quantities for mixture divided by the mass of dry air and trace gas only, as opposed to dividing by the mass of the whole mixture. The whole mixture includes dry air, water vapor, and trace gas.

Two-Phase Fluid Heat Transfer Correlation

If the segment is subcooled liquid, then the heat transfer conductance is

UAseg,L,2P=aL,2P(Reseg,L,2P)b2P(Prseg,L,2P)c2Pkseg,L,2PG2PN,

where:

  • aL,2P, b2P, and c2P are the coefficients of the Nusselt number correlation. These coefficients appear as block parameters in the Correlation Coefficients section.

  • Reseg,L,2P is the average liquid Reynolds number for the segment.

  • Prseg,L,2P is the average liquid Prandtl number for the segment.

  • kseg,L,2P is the average liquid thermal conductivity for the segment.

  • G2P is the geometry scale factor for the two-phase fluid side of the heat exchanger. The block calculates the geometry scale factor so that the total heat transfer over all segments matches the specified performance at the nominal operating conditions.

The average liquid Reynolds number is

Reseg,L,2P=m˙seg,2PDref,2Pμseg,L,2PSref,2P,

where:

  • m˙seg,2P is the mass flow rate through the segment.

  • μseg,L,2P is the average liquid dynamic viscosity for the segment.

  • Dref,2P is an arbitrary reference diameter.

  • Sref,2P is an arbitrary reference flow area.

Note

The Dref,2P and Sref,2P terms are included in this equation for unit calculation purposes only, to make Reseg,L,2P nondimensional. The values of Dref,2P and Sref,2P are arbitrary because the G2P calculation overrides these values.

Similarly, if the segment is superheated vapor, then the heat transfer conductance is

UAseg,V,2P=aV,2P(Reseg,V,2P)b2P(Prseg,V,2P)c2Pkseg,V,2PG2PN,

where:

  • aV,2P, b2P, and c2P are the coefficients of the Nusselt number correlation. These coefficients appear as block parameters in the Correlation Coefficients section.

  • Reseg,V,2P is the average vapor Reynolds number for the segment.

  • Prseg,V,2P is the average vapor Prandtl number for the segment.

  • kseg,V,2P is the average vapor thermal conductivity for the segment.

The average vapor Reynolds number is

Reseg,V,2P=m˙seg,2PDref,2Pμseg,V,2PSref,2P,

where μseg,V,2P is the average vapor dynamic viscosity for the segment.

If the segment is liquid-vapor mixture, then the heat transfer conductance is

UAseg,M,2P=aM,2P(Reseg,SL,2P)b2PCZ(Prseg,SL,2P)c2Pkseg,SL,2PG2PN,

where:

  • aM,2P, b2P, and c2P are the coefficients of the Nusselt number correlation. These coefficients appear as block parameters in the Correlation Coefficients section.

  • Reseg,SL,2P is the saturated liquid Reynolds number for the segment.

  • Prseg,SL,2P is the saturated liquid Prandtl number for the segment.

  • kseg,SL,2P is the saturated liquid thermal conductivity for the segment.

  • CZ is the Cavallini and Zecchin term.

The saturated liquid Reynolds number is

Reseg,SL,2P=m˙seg,2PDref,2Pμseg,SL,2PSref,2P,

where μseg,SL,2P is the saturated liquid dynamic viscosity for the segment.

The Cavallini and Zecchin term is

CZ=((νseg,SV,2Pνseg,SL,2P1)(xseg,out,2P+1))1+b2P((νseg,SV,2Pνseg,SL,2P1)(xseg,in,2P+1))1+b2P(1+b2P)(νseg,SV,2Pνseg,SL,2P1)(xseg,out,2Pxseg,in,2P),

where:

  • νseg,SL,2P is the saturated liquid specific volume for the segment.

  • νseg,SV,2P is the saturated vapor specific volume for the segment.

  • xseg,in,2P is the vapor quality at the segment inlet.

  • xseg,out,2P is the vapor quality at the segment outlet.

The expression is based on the work of Cavallini and Zecchin [5], which derives a heat transfer coefficient correlation at a local vapor quality x. Equations for the liquid-vapor mixture are obtained by averaging Cavallini and Zecchin’s correlation over the segment from xseg,in,2P to xseg,out,2P.

Two-Phase Fluid Weighted Average

The two-phase fluid flow through a segment may not be entirely represented as either subcooled liquid, superheated vapor, or liquid-vapor mixture. Instead, each segment may consist of a combination of these. The block approximates this condition by computing weighting factors based on the change in specific enthalpy across the segment and the saturated liquid and vapor specific enthalpies:

wL=|min(hseg,out,2P,hseg,SL,2P)min(hseg,in,2P,hseg,SL,2P)||hseg,out,2Phseg,in,2P|wV=|max(hseg,out,2P,hseg,SV,2P)max(hseg,in,2P,hseg,SV,2P)||hseg,out,2Phseg,in,2P|wM=1wLwV

where:

  • hseg,in,2P is the specific enthalpy at the segment inlet.

  • hseg,out,2P is the specific enthalpy at the segment outlet.

  • hseg,SL,2P is the saturated liquid specific enthalpy for the segment.

  • hseg,SV,2P is the saturated vapor specific enthalpy for the segment.

The weighted-average two-phase fluid heat transfer conductance for the segment is therefore

UAseg,2P=wL(UAseg,L,2P)+wV(UAseg,V,2P)+wM(UAseg,M,2P).

The weighted-average fluid temperature for the segment is

Tseg,2P=wL(UAseg,L,2P)Tseg,L,2P+wV(UAseg,V,2P)Tseg,V,2P+wM(UAseg,M,2P)Tseg,M,2PUAseg,2P,

where:

  • Tseg,L,2P is the average liquid temperature for the segment.

  • Tseg,V,2P is the average vapor temperature for the segment.

  • Tseg,M,2P is the average mixture temperature for the segment, which is the saturated liquid temperature.

Moist Air Heat Transfer Correlation

The heat transfer conductance is

UAseg,MA=aMA(Reseg,MA)bMA(Prseg,MA)cMAkseg,MAGMAN,

where:

  • aMA, bMA, and cMA are the coefficients of the Nusselt number correlation. These coefficients appear as block parameters in the Correlation Coefficients section.

  • Reseg,MA is the average Reynolds number for the segment.

  • Prseg,MA is the average Prandtl number for the segment.

  • kseg,MA is the average thermal conductivity for the segment.

  • GMA is the geometry scale factor for the moist air side of the heat exchanger. The block calculates the geometry scale factor so that the total heat transfer over all segments matches the specified performance at the nominal operating conditions.

The average Reynolds number is

Reseg,MA=m˙seg,MADref,MAμseg,MASref,MA,

where:

  • m˙seg,MA is the mass flow rate through the segment.

  • μseg,MA is the average dynamic viscosity for the segment.

  • Dref,MA is an arbitrary reference diameter.

  • Sref,MA is an arbitrary reference flow area.

Note

The Dref,MA and Sref,MA terms are included in this equation for unit calculation purposes only, to make Reseg,MA nondimensional. The values of Dref,MA and Sref,MA are arbitrary because the GMA calculation overrides these values.

Moist Air Condensation

The equation describing the heat flow rate from the wall to the moist air in the segment (the last equation in the Heat Transfer section) uses the average moist air mixture enthalpy, h¯seg,MA, and the wall segment moist air mixture enthalpy, h¯seg,wall.

The average moist air mixture enthalpy is based on the temperature and humidity of the moist air flow through the segment:

h¯seg,MA=hseg,ag,MA+Wseg,MAhseg,w,MA,

where:

  • hseg,ag,MA is the average specific enthalpy of dry air and trace gas for the segment.

  • hseg,w,MA is the average specific enthalpy of water vapor for the segment.

  • Wseg,MA is the humidity ratio of the segment.

The wall segment moist air mixture enthalpy is based on the temperature and humidity at the wall segment:

h¯seg,wall=hseg,ag,wall+Wseg,wallhseg,w,wall,

where:

  • hseg,ag,wall is the specific enthalpy of dry air and trace gas at the wall segment temperature.

  • hseg,w,wall is the specific enthalpy of water vapor at the wall segment temperature.

  • Wseg,wall is the humidity ratio at the wall segment:

    Wseg,wall=min(Wseg,MA,Wseg,s,wall),

    where Wseg,s,wall is the saturated humidity ratio at the wall segment temperature. In other words, the humidity ratio at the wall is the same as the humidity ratio of the moist air flow but not more than the maximum that can be supported at the wall segment temperature.

When Wseg,s,wall < Wseg,MA, water vapor condensation occurs on the wall surface. The rate of water vapor condensation is

m˙w,seg,cond=UAseg,MAc¯pseg,MA(Wseg,MAWseg,wall).

The condensed water is assumed to be drained from the wall surface and is thus removed from the moist air flow downstream.

Pressure Loss

The pressure losses on the two-phase fluid side are

pA,2Pp2P=K2P2m˙A,2Pm˙2A,2P+m˙2thres,2P2ρavg,2PpB,2Pp2P=K2P2m˙B,2Pm˙2B,2P+m˙2thres,2P2ρavg,2P

where:

  • pA,2P and pB,2P are the pressures at ports A1 and B1, respectively.

  • p2P is internal two-phase fluid pressure at which the heat transfer is calculated.

  • m˙A,2P and m˙B,2P are the mass flow rates into ports A1 and B1, respectively.

  • ρavg,2P is the average two-phase fluid density over all segments.

  • m˙thres,2P is the laminar threshold for pressure loss, approximated as 1e-4 of the nominal mass flow rate. The block calculates the pressure loss coefficient, K2P, so that pA,2PpB,2P matches the nominal pressure loss at the nominal mass flow rate.

The pressure losses on the moist air side are

pA,MApMA=KMA2m˙A,MAm˙2A,MA+m˙2thres,MA2ρavg,2PpB,MApMA=KMA2m˙B,MAm˙2B,MA+m˙2thres,MA2ρavg,MA

where:

  • pA,MA and pB,MA are the pressures at ports A2 and B2, respectively.

  • pMA is internal moist air pressure at which the heat transfer is calculated.

  • m˙A,MA and m˙B,MA are the mass flow rates into ports A2 and B2, respectively.

  • ρavg,MA is the average moist air density over all segments.

  • m˙thres,MA is the laminar threshold for pressure loss, approximated as 1e-4 of the nominal mass flow rate. The block calculates the pressure loss coefficient, KMA, so that pA,MApB,MA matches the nominal pressure loss at the nominal mass flow rate.

Two-Phase Fluid Mass and Energy Conservation

The mass conservation equation for the overall two-phase fluid flow is

(dp2Pdtsegments(ρseg,2Pp)+segments(duseg,2Pdtρseg,2Pu))V2PN=m˙A,2P+m˙B,2P,

where:

  • ρseg,2Pp is the partial derivative of density with respect to pressure for the segment.

  • ρseg,2Pu is the partial derivative of density with respect to specific internal energy for the segment.

  • useg,2P is the specific internal energy for the segment.

  • V2P is the total two-phase fluid volume.

The summation is over all segments.

Note

Although the two-phase fluid flow is divided into N=3 segments for heat transfer calculations, all segments are assumed to be at the same internal pressure, p2P. That is why p2P is outside of the summation.

The energy conservation equation for each segment is

duseg,2PdtM2PN+useg,2P(m˙seg,in,2Pm˙seg,out,2P)=Φseg,in,2PΦseg,out,2P+Qseg,2P,

where:

  • M2P is the total two-phase fluid mass.

  • m˙seg,in,2P and m˙seg,out,2P are the mass flow rates into and out of the segment.

  • Φseg,in,2p and Φseg,out,2p are the energy flow rates into and out of the segment.

The mass flow rates between segments are assumed to be linearly distributed between the values ofm˙A,2P and m˙B,2P.

Moist Air Mass and Energy Conservation

The mass conservation for the overall moist air mixture flow is

(dpMAdtsegments(ρseg,MAp)+segments(dTseg,MAdtρseg,MAT+dxw,seg,MAdtρseg,MAxw+dxg,seg,MAdtρseg,MAxg))VMAN=m˙A,MA+m˙B,MAsegments(m˙w,seg,cond),

where:

  • ρseg,MAp is the partial derivative of density with respect to pressure for the segment.

  • ρseg,MAT is the partial derivative of density with respect to temperature for the segment.

  • ρseg,MAxw is the partial derivative of density with respect to specific humidity for the segment.

  • ρseg,MAxg is the partial derivative of density with respect to trace gas mass fraction for the segment.

  • xw,seg,MA is the specific humidity, that is, the water vapor mass fraction, for the segment.

  • xg,seg,MA is the trace gas mass fraction for the segment.

  • VMA is the total moist air volume.

The summation is over all segments.

Note

Although the two-phase fluid flow is divided into N=3 segments for heat transfer calculations, all segments are assumed to be at the same internal pressure, pMA. That is why pMA is outside of the summation.

The energy conservation equation for each segment is

(dTseg,MAdtuseg,MAT+dxw,seg,MAdtuseg,MAxw+dxg,seg,MAdtuseg,MAxg)MMAN+useg,MA(m˙seg,in,MAm˙seg,out,MA)=Φseg,in,MAΦseg,out,MA+Qseg,MAm˙w,seg,condhl,wall,

where:

  • useg,MAT is the partial derivative of specific internal energy with respect to temperature for the segment.

  • useg,MAxw is the partial derivative of specific internal energy with respect to specific humidity for the segment.

  • useg,MAxg is the partial derivative of specific internal energy with respect to trace gas mass fraction for the segment.

  • useg,2P is the specific internal energy for the segment.

  • MMA is the total moist air mass.

  • m˙seg,in,MA and m˙seg,out,MA are the mass flow rates into and out of the segment.

  • Φseg,in,MA and Φseg,out,MA are the energy flow rates into and out of the segment.

The mass flow rates between segments are assumed to be linearly distributed between the values ofm˙A,MA and m˙B,MA.

The water vapor mass conservation equation for each segment is

dxw,seg,MAdtMMAN+xw,seg,MA(m˙seg,in,MAm˙seg,out,MA)=m˙w,seg,in,MAm˙w,seg,out,MAm˙w,seg,cond,

where m˙w,seg,in,MA and m˙w,seg,out,MA are the water vapor mass flow rates into and out of the segment.

The trace gas mass conservation equation for each segment is

dxg,seg,MAdtMMAN+xg,seg,MA(m˙seg,in,MAm˙seg,out,MA)=m˙g,seg,in,MAm˙g,seg,out,MA,

where m˙g,seg,in,MA and m˙g,seg,out,MA are the trace gas mass flow rates into and out of the segment.

Ports

Output

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Rate of heat transfer to two-phase fluid, returned as a physical signal. Physical signals Q1 and Q2 are usually equal in value with opposite sign. However, if the Wall thermal mass parameter is set to On, then these two signals may have different values because the wall may absorb and release some of the heat being transferred.

Rate of heat transfer to moist air, returned as a physical signal. Physical signals Q1 and Q2 are usually equal in value with opposite sign. However, if the Wall thermal mass parameter is set to On, then these two signals may have different values because the wall may absorb and release some of the heat being transferred.

Water condensation rate in the moist air flow, returned as a physical signal. The condensate does not accumulate on the heat transfer surface.

Conserving

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Inlet or outlet port associated with the two-phase fluid.

Inlet or outlet port associated with the two-phase fluid.

Inlet or outlet port associated with the moist air.

Inlet or outlet port associated with the moist air.

Parameters

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Configuration

Enable or disable the effect of thermal mass on the heat transfer surface. Setting this parameter to On introduces additional dynamics to the simulation, so that it takes longer to reach steady state, but does not affect the results at steady-state simulation.

Mass of the heat transfer surface.

Dependencies

To enable this parameter, set Wall thermal mass to On.

Specific heat of the heat transfer surface.

Dependencies

To enable this parameter, set Wall thermal mass to On.

Flow area at the two-phase fluid port A1.

Flow area at the two-phase fluid port B1.

Flow area at the moist air port A2.

Flow area at the moist air port B2.

Two-Phase Fluid 1

Select the nominal operating condition:

  • Condenser - heat transfer from two-phase fluid to moist air — The two-phase fluid is being cooled and the moist air is being heated.

  • Evaporator - heat transfer from moist air to two-phase fluid — The moist air is being cooled and the two-phase fluid is being heated.

This choice relates only to specifying the nominal operating condition parameters. It does not mean that heat transfer can only happen in the specified direction during simulation.

Mass flow rate from port A1 to port B1 during the nominal operating condition.

Pressure drop from port A1 to port B1 during the nominal operating condition.

Select the method of pressure specification:

  • Inlet pressure — Specify the nominal inlet pressure.

  • Saturation pressure at specified condensing temperature — Specify the nominal condensing temperature. This option is available if you set Nominal operating condition to Condenser - heat transfer from two-phase fluid to moist air.

  • Saturation pressure at specified evaporating temperature — Specify the nominal evaporating temperature. This option is available if you set Nominal operating condition to Evaporator - heat transfer from moist air to two-phase fluid.

Pressure at the inlet of the two-phase fluid side of the heat exchanger during nominal operating condition.

Dependencies

To enable this parameter, set Pressure specification to Inlet pressure.

Liquid saturation temperature at the outlet of the two-phase fluid side of the heat exchanger during nominal operating condition. The pressure in the heat exchanger is the corresponding saturation pressure. The nominal condensing temperature must be less than the critical temperature.

Dependencies

To enable this parameter, set Nominal operating condition to Condenser - heat transfer from two-phase fluid to moist air and Pressure specification to Saturation pressure at specified condensing temperature.

Vapor saturation temperature at the outlet of the two-phase fluid side of the heat exchanger during nominal operating condition. The pressure in the heat exchanger is the corresponding saturation pressure. The nominal evaporating temperature must be less than the critical temperature.

Dependencies

To enable this parameter, set Nominal operating condition to Evaporator - heat transfer from moist air to two-phase fluid and Pressure specification to Saturation pressure at specified evaporating temperature.

Quantity used to describe the inlet condition of the fluid at the nominal operating condition: temperature, specific enthalpy, or vapor quality.

Specific enthalpy at the inlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet condition specification to Specific enthalpy.

Temperature at the inlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet condition specification to Temperature.

Vapor quality, defined as the mass fraction of vapor in a liquid-vapor mixture, at the inlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet condition specification to Vapor quality.

Select whether to specify the performance of the heat exchanger at the nominal operating condition directly, by the rate of heat transfer, or indirectly, by the outlet condition.

Rate of heat transfer, depending on the nominal operating condition:

  • If Nominal operating condition is Condenser - heat transfer from two-phase fluid to moist air, rate of heat transfer from the two-phase fluid to the moist air during the nominal operating condition.

  • If Nominal operating condition is Evaporator - heat transfer from moist air to two-phase fluid , rate of heat transfer from the moist air to the two-phase fluid during the nominal operating condition.

Dependencies

To enable this parameter, set Heat transfer capacity specification to Rate of heat transfer.

Select the quantity for outlet condition specification:

  • Specific enthalpy — Specify the nominal specific enthalpy.

  • Subcooling — Specify the nominal subcooling. This option is available if you set Nominal operating condition to Condenser - heat transfer from two-phase fluid to moist air.

  • Superheating — Specify the nominal superheating. This option is available if you set Nominal operating condition to Evaporator - heat transfer from moist air to two-phase fluid.

  • Vapor quality — Specify the nominal vapor quality.

Dependencies

To enable this parameter, set Heat transfer capacity specification to Outlet condition.

Specific enthalpy at the outlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Outlet condition specification to Specific enthalpy.

Degree of temperature below the liquid saturation temperature at the outlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Outlet condition specification to Subcooling.

Degree of temperature above the vapor saturation temperature at the outlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Outlet condition specification to Superheating.

Two-phase fluid vapor quality, defined as the mass fraction of vapor in a liquid-vapor mixture, at the outlet of the two-phase fluid side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Outlet condition specification to Vapor quality.

Total volume of two-phase fluid inside the heat exchanger.

Select how to specify initial state of two-phase fluid:

  • Same as nominal operating condition — Start the simulation at the nominal operating condition.

  • Specify initial condition — Specify a different set of initial conditions using additional parameters.

Quantity used to describe the initial state of the fluid: temperature, vapor quality, vapor void fraction, specific enthalpy, or specific internal energy.

Dependencies

To enable this parameter, set Initial condition specification to Specify initial condition.

Two-phase fluid pressure at the start of the simulation.

Two-phase fluid specific enthalpy at the start of simulation. If the value is a scalar, then the initial specific enthalpy is assumed uniform. If the value is a two-element vector, then the initial specific enthalpy is assumed to vary linearly between ports A1 and B1, with the first element corresponding to port A1 and the second element corresponding to port B1.

Dependencies

To enable this parameter, set Initial fluid energy specification to Specific enthalpy.

Two-phase fluid temperature at the start of simulation. If the value is a scalar, then the initial temperature is assumed uniform. If the value is a two-element vector, then the initial temperature is assumed to vary linearly between ports A1 and B1, with the first element corresponding to port A1 and the second element corresponding to port B1.

Dependencies

To enable this parameter, set Initial fluid energy specification to Temperature.

Two-phase fluid vapor quality, defined as the mass fraction of vapor in a liquid-vapor mixture, at the start of simulation. If the value is a scalar, then the initial vapor quality is assumed uniform. If the value is a two-element vector, then the initial vapor quality is assumed to vary linearly between ports A1 and B1, with the first element corresponding to port A1 and the second element corresponding to port B1.

If using this option, the initial pressure cannot be higher than the critical pressure.

Dependencies

To enable this parameter, set Initial fluid energy specification to Vapor quality.

Two-phase fluid vapor void fraction, defined as the volume fraction of vapor in a liquid-vapor mixture,at the start of simulation. If the value is a scalar, then the initial vapor void fraction is assumed uniform. If the value is a two-element vector, then the initial vapor void fraction is assumed to vary linearly between ports A1 and B1, with the first element corresponding to port A1 and the second element corresponding to port B1.

If using this option, the initial pressure cannot be higher than the critical pressure.

Dependencies

To enable this parameter, set Initial fluid energy specification to Vapor void fraction.

Two-phase fluid specific internal energy at the start of simulation. If the value is a scalar, then the initial specific internal energy is assumed uniform. If the value is a two-element vector, then the initial specific internal energy is assumed to vary linearly between ports A1 and B1, with the first element corresponding to port A1 and the second element corresponding to port B1.

Dependencies

To enable this parameter, set Initial fluid energy specification to Specific internal energy.

Moist Air 2

Mass flow rate from port A2 to port B2 during the nominal operating condition.

Pressure drop from port A2 to port B2 during the nominal operating condition.

Pressure at the inlet of the moist air side of the heat exchanger during nominal operating condition.

Temperature at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

Select quantity used to describe the moisture level at the inlet during the nominal operating condition: relative humidity, specific humidity, water vapor mole fraction, or humidity ratio.

Relative humidity at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet moisture specification to Relative humidity.

Specific humidity, defined as the mass fraction of water vapor in a moist air mixture, at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet moisture specification to Specific humidity.

Mole fraction of water vapor in a moist air mixture at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet moisture specification to Mole fraction.

Humidity ratio, defined as the mass ratio of water vapor to dry air and trace gas, at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

Dependencies

To enable this parameter, set Inlet moisture specification to Humidity ratio.

Select quantity used to describe the trace gas level at the inlet during the nominal operating condition: mass fraction or mole fraction.

Mass fraction of trace gas in a moist air mixture at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

This parameter is ignored if the Trace gas model parameter in the Moist Air Properties (MA) block is set to None.

Dependencies

To enable this parameter, set Inlet trace gas specification to Mass fraction.

Mole fraction of trace gas in a moist air mixture at the inlet of the moist air side of the heat exchanger during the nominal operating condition.

This parameter is ignored if the Trace gas model parameter in the Moist Air Properties (MA) block is set to None.

Dependencies

To enable this parameter, set Inlet trace gas specification to Mole fraction.

Total volume of moist air in the heat exchanger.

Select how to specify initial state of moist air:

  • Same as nominal operating condition — Start the simulation at the nominal operating condition.

  • Specify initial condition — Specify a different set of initial conditions using additional parameters.

Moist air pressure at the start of the simulation.

Moist air temperature at the start of simulation. If the value is a scalar, then the initial temperature is assumed uniform. If the value is a two-element vector, then the initial temperature is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

Select quantity used to describe the initial moisture level: relative humidity, specific humidity, water vapor mole fraction, or humidity ratio.

Moist air relative humidity at the start of simulation. If the value is a scalar, then the initial relative humidity is assumed uniform. If the value is a two-element vector, then the initial relative humidity is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

Dependencies

To enable this parameter, set Initial moisture specification to Relative humidity.

Moist air specific humidity, defined as the mass fraction of water vapor in a moist air mixture, at the start of simulation. If the value is a scalar, then the initial specific humidity is assumed uniform. If the value is a two-element vector, then the initial specific humidity is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

Dependencies

To enable this parameter, set Initial moisture specification to Specific humidity.

Mole fraction of water vapor in a moist air mixture at the start of simulation. If the value is a scalar, then the initial mole fraction is assumed uniform. If the value is a two-element vector, then the initial mole fraction is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

Dependencies

To enable this parameter, set Initial moisture specification to Mole fraction.

Moist air humidity ratio, defined as the mass ratio of water vapor to dry air and trace gas, at the start of simulation. If the value is a scalar, then the initial humidity ratio is assumed uniform. If the value is a two-element vector, then the initial humidity ratio is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

Dependencies

To enable this parameter, set Initial moisture specification to Humidity ratio.

Select quantity used to describe the trace gas level at the start of simulation: mass fraction or mole fraction.

Mass fraction of trace gas in a moist air mixture at the start of simulation. If the value is a scalar, then the initial mass fraction is assumed uniform. If the value is a two-element vector, then the initial mass fraction is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

This parameter is ignored if the Trace gas model parameter in the Moist Air Properties (MA) block is set to None.

Dependencies

To enable this parameter, set Initial trace gas specification to Mass fraction.

Mole fraction of trace gas in a moist air mixture at the start of simulation. If the value is a scalar, then the initial mole fraction is assumed uniform. If the value is a two-element vector, then the initial mole fraction is assumed to vary linearly between ports A2 and B2, with the first element corresponding to port A2 and the second element corresponding to port B2.

This parameter is ignored if the Trace gas model parameter in the Moist Air Properties (MA) block is set to None.

Dependencies

To enable this parameter, set Initial trace gas specification to Mole fraction.

Relative humidity point of condensation. Condensation occurs above this value. In most cases, this value is 1, that is, 100%. A value greater than 1 indicates a supersaturated vapor.

Correlation Coefficients

Proportionality constant in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for subcooled liquid in two-phase fluid. The default value is based on the Colburn equation.

Proportionality constant in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for liquid-vapor mixture in two-phase fluid. The default value is based on the Cavallini and Zecchin correlation.

Proportionality constant in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for superheated vapor in two-phase fluid. The default value is based on the Colburn equation.

Reynolds number exponent in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for two-phase fluid. The same value applies to subcooled liquid, liquid-vapor mixture, and superheated vapor.

Prandtl number exponent in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for two-phase fluid. The same value applies to subcooled liquid, liquid-vapor mixture, and superheated vapor.

Proportionality constant in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for moist air. The default value is based on the Colburn equation.

Reynolds number exponent in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for moist air. The default value is based on the Colburn equation.

Prandtl number exponent in the correlation of Nusselt number as a function of Reynolds number and Prandtl number for moist air. The default value is based on the Colburn equation.

References

[1] Ashrae Handbook: Fundamentals. Atlanta: Ashrae, 2013.

[2] Çengel, Yunus A. Heat and Mass Transfer: A Practical Approach. 3rd ed. McGraw-Hill Series in Mechanical Engineering. Boston: McGraw-Hill, 2007.

[3] Mitchell, John W., and James E. Braun. Principles of Heating, Ventilation, and Air Conditioning in Buildings. Hoboken, NJ: Wiley, 2013.

[4] Shah, R. K., and Dušan P. Sekulić. Fundamentals of Heat Exchanger Design. Hoboken, NJ: John Wiley & Sons, 2003.

[5] Cavallini, Alberto, and Roberto Zecchin. “A DIMENSIONLESS CORRELATION FOR HEAT TRANSFER IN FORCED CONVECTION CONDENSATION.” In Proceeding of International Heat Transfer Conference 5, 309–13. Tokyo, Japan: Begellhouse, 1974. https://doi.org/10.1615/IHTC5.1220.

Extended Capabilities

C/C++ Code Generation
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Introduced in R2021b