ENTU Heat Transfer
Detailed heat transfer model between two general fluids
Libraries:
Simscape /
Fluids /
Heat Exchangers /
Fundamental Components
Description
The ENTU Heat Transfer block models the heat exchange between two general fluids based on the standard EffectivenessNTU method. The fluid thermal properties are specified explicitly through Simscape™ physical signals. Combine with the Heat Exchanger Interface (TL) or Heat Exchanger Interface (G) blocks to model the pressure drop and temperature change between the inlet and outlet of a heat exchanger.
The block dialog box provides a choice of common heat exchanger configurations. These include concentricpipe with parallel and counter flows, shellandtube with one or more shell passes, and crossflow with mixed and unmixed flows. A generic configuration lets you model other heat exchangers based on tabular effectiveness data.
Heat Exchanger Configurations
Heat Transfer Rate
The ENTU model defines the heat transfer rate between fluids 1 and 2 in terms of an effectiveness parameter ε
$$\begin{array}{cc}{Q}_{1}={Q}_{2}=\u03f5{Q}_{Max},& 0<\epsilon <1\end{array},$$
where:
Q_{1} and Q_{2} are the heat transfer rates into fluid 1 and fluid 2.
Q_{Max} is the maximum possible heat transfer rate between fluid 1 and fluid 2 at a given set of operating conditions.
ε is the effectiveness parameter.
The maximum possible heat transfer rate between the two fluids is
$${Q}_{Max}={C}_{Min}\left({T}_{1,In}{T}_{2,In}\right),$$
where:
C_{Min} is the minimum value of the thermal capacity rate:
$${C}_{Min}=min\left({\dot{m}}_{1}{c}_{p,1},{\dot{m}}_{2}{c}_{p,2}\right)$$
T_{1,In} and T_{2,In} are the inlet temperatures of fluid 1 and fluid 2.
$${\dot{m}}_{1}$$ and $${\dot{m}}_{2}$$ are the mass flow rates of fluid 1 and fluid 2 into the heat exchanger volume through the inlet.
c_{p,1} and c_{p,2} are the specific heat coefficients at constant pressure of fluid 1 and fluid 2. The Minimum fluidwall heat transfer coefficient parameter in the block dialog box sets a lower bound on the allowed values of the heat transfer coefficients.
Heat Exchanger Effectiveness
The heat exchanger effectiveness calculations depend on the flow arrangement type
selected in the block dialog box. For all but Generic —
effectiveness table
, the block computes the thermal exchange
effectiveness through analytical expressions written in terms of the number of
transfer units (NTU) and thermal capacity ratio. The number of transfer units is
defined as
$$NTU=\frac{{U}_{Overall}{A}_{Heat}}{{C}_{Min}}=\frac{1}{{C}_{Min}{R}_{Overall}},$$
where:
NTU is the number of transfer units.
U_{Overall} is the overall heat transfer coefficient between fluid 1 and fluid 2.
R_{Overall} is the overall thermal resistance between fluid 1 and fluid 2.
A_{Heat} is aggregate area of the primary and secondary, or finned, heat transfer surfaces.
The thermal capacity ratio is defined as
$${C}_{rel}=\frac{{C}_{Min}}{{C}_{Max}}$$
where:
C_{rel} is the thermal capacity ratio.
The overall heat transfer coefficient and thermal resistance used in the NTU calculation are functions of the heat transfer mechanisms at work. These mechanisms include convective heat transfer between the fluids and the heat exchanger interface and conduction through the interface wall [2]:
$${R}_{Overall}=\frac{1}{{U}_{Overall}{A}_{Heat}}=\frac{1}{{h}_{1}{A}_{Heat,1}}+{R}_{Foul,1}+{R}_{Wall}+{R}_{Foul,2}+\frac{1}{{h}_{2}{A}_{Heat,2}},$$
where:
h_{1} and h_{2} are the heat transfer coefficients between fluid 1 and the interface wall and between fluid 2 and the interface wall.
A_{Heat,1} and A_{Heat,2} are the heat transfer surface areas on the fluid1 and fluid2 sides.
R_{Foul,1} and R_{Foul,2} are the fouling resistances on the fluid1 and fluid2 sides. The fouling resistance is equal to the fouling factor parameter divided by the heat transfer surface area.
R_{Wall} is the interface wall thermal resistance.
Heat Transfer From Fluid 1 to Fluid 2
The tables show some of the analytical expressions used to compute the heat exchange effectiveness [1]. The parameter N refers to the number of shell passes and the parameter ε_{1} to the effectiveness for a single shell pass.
Concentric Tubes  
Counter Flow 
$$\epsilon =\{\begin{array}{cc}\frac{1\mathrm{exp}\left[NTU\left(1{C}_{rel}\right)\right]}{1{C}_{rel}\mathrm{exp}\left[NTU\left(1{C}_{rel}\right)\right]},& \text{if}{C}_{rel}1\\ \frac{NTU}{1+NTU},& \text{if}{C}_{rel}=1\end{array}$$

Parallel Flow 
$$\epsilon =\frac{1\mathrm{exp}\left[NTU\left(1+{C}_{rel}\right)\right]}{1+{C}_{rel}}$$

Shell and Tube  
One shell pass and two, four, or six tube passes 
$${\epsilon}_{1}=\frac{2}{1+{C}_{rel}+\sqrt{1+{C}_{rel}{}^{2}}\frac{1+\mathrm{exp}\left(NTU\sqrt{1+{C}_{rel}{}^{2}}\right)}{1\mathrm{exp}\left(NTU\sqrt{1+{C}_{rel}{}^{2}}\right)}}$$

N Shell Passes and 2N, 4N, or 6N Tube Passes 
$$\epsilon =\frac{{\left[\left(1{\epsilon}_{1}{C}_{rel}\right)/\left(1{\epsilon}_{1}\right)\right]}^{N}1}{{\left[\left(1{\epsilon}_{1}{C}_{rel}\right)/\left(1{\epsilon}_{1}\right)\right]}^{N}{C}_{rel}}$$

Cross Flow (Single Pass)  
Both Fluids Unmixed 
$$\epsilon =1\mathrm{exp}\left(\frac{\mathrm{exp}\left({C}_{rel}NT{U}^{0.78}\right)1}{{C}_{rel}NT{U}^{0.22}}\right)$$

Both Fluids Mixed 
$$\epsilon =\frac{1}{\frac{1}{1exp\left(NTU\right)}+\frac{{C}_{rel}}{1\mathrm{exp}\left({C}_{rel}NTU\right)}\frac{1}{NTU}}$$

C_{Max} mixed, C_{Min} unmixed 
$$\epsilon =\frac{1}{{C}_{rel}}\left(1\mathrm{exp}\left({C}_{rel}\left(1\mathrm{exp}\left(NTU\right)\right)\right)\right)$$

C_{Max} unmixed, C_{Min} mixed 
$$\epsilon =1\mathrm{exp}\left(\frac{1}{{C}_{rel}}\left(1\mathrm{exp}\left({C}_{rel}NTU\right)\right)\right)$$

Assumptions and Limitations
The flows are singlephase. The heat transfer is strictly one of sensible heat. The transfer is limited to interior of the exchanger, with the environment neither gaining heat from nor providing heat to the flows—the heat exchanger is an adiabatic component.
Ports
Input
Conserving
Parameters
References
[1] Holman, J. P. Heat Transfer. 9th ed. New York, NY: McGraw Hill, 2002.
[2] Shah, R. K. and D. P. Sekulic. Fundamentals of Heat Exchanger Design. Hoboken, NJ: John Wiley & Sons, 2003.
Extended Capabilities
Version History
Introduced in R2016a