Macro Heat Exchanger Model Theory

Some salient features of Macro Heat Exchanger Model Theory.

Macro Heat Exchanger Model Theory
A. ***************
1. Heat Exchanger Performance Curve: NTU_full (mdot_air_full)
2. NTU-E Curve
Even if full exchanger is multi-pass, a macro is always single-pass. So NTU-E of
single-pass can be applied on a heat exchanger that has multiple passes.

NTU - Number of Transfer Units
E - Effectiveness

Determining NTU_macro
The NTU_full (mdot_air_full) curve is for the full heat exchanger and does not
directly apply to the macro. However, using simple ratios, we can relate
NTU_macro to the NTU_full curve.

NTU_macro = NTU_full * A_macro/A_full * Cmin_full/Cmin_macro

where A is the surface area at which heat transfer takes place, which would be
the fins, tubes, etc. For homogeneous core, the ratio of the areas has to equal
the ratio of the volumes: A_macro/A_full = V_macro/V_full.

Also, NTU_full has to be evaluated at local flow rate (mdot_macro *
Af_total/Af_full), where Af is the frontal inlet let area.

NTU_macro = NTU_full * V_macro/V_full * Cmin_full/Cmin_macro

NTU_macro = V_macro/V_full * Cmin_full/Cmin_macro * NTU_full

C. ***************
Given NTU_macro, calculate effectiveness of the macro using NTU-effectiveness

E_macro = f (NTU_macro, R_macro), where R_macro =

D. ***************
Calculate heat rejection of the macro.
Q_macro = Cmin_macro * E_macro * ( Tcoolant-in_macro - Tair-in_macro)

E. ***************
Calculate heat rejection/added for each cell within a macro.
Q_cell = Q_macro * Vcell/Vmacro
where Vcell, and Vmacro are volumes of cell and macro, respectively.


-To properly account for variable density, mdot, not velocity, is used.

-The above assumes fixed coolant flow rate.

-Only one performance curve is used for a single coolant flow-rate is used.

-If it is a multi-pass heat exchanger, use multi-pass NTU-E relation to
convert raw heat exchanger data into NTU_full (mdot_air_full) for Step A.

-The macro always uses single-pass NTU-E relation, where both fluid are unmixed.
The curve is:
E = 1-exp[(1/Cr)(NTU)^.22{exp[-Cr(NTU)^.78] - 1}],

Show Form
No comments yet. Be the first to add a comment!