Thermal performance modelling of a spray cooled heat exchanger using cell-based NTU method

Kurzfassung

Spray cooled heat exchangers (HEXs) that utilize evaporative cooling of the thin liquid film sprayed on to the air-side heat transfer surface area offer significantly enhanced thermal performance compared to conventional dry heat exchangers. However, accurately quantifying this heat transfer is challenging due to the simultaneous heat and mass transfer processes involved in liquid-gas phase change. The thermal performance prediction becomes more complex when multi-pass compact heat exchangers are involved, which is often the case in practical applications. In this thesis, a semi-empirical solver model has been developed for the analysis of spray cooled multi-pass HEXs. The solver predicts the thermal behavior of a HEX placed downstream of a horizontal spray cooling system. The model is capable of handling different surface wetting conditions including fully dry, fully wet and semiwet conditions. The approach combines the conventional effectiveness-NTU method for singlephase cross-flow HEX design with the governing equations for the thermophysical processes of sub-saturation liquid film evaporation. The solver adopts a cell method to discretize the HEX into smaller interconnected elements, each acting as a standalone HEX with simple cross flow, parallel flow or counter flow arrangements. This enabled the use of effectiveness-NTU equations for the heat calculations available in known literature. The overall HEX flow arrangement, such as cross-counter flow or cross-parallel flow, was realized through the cell interconnections. For the cell elements with wetted surfaces, the effectiveness{NTU method was adapted to an enthalpy-based formulation, since the enthalpy of moist air captures both the temperature variation and the mass transfer between the liquid film and the air stream. To unify all heat transfer mechanisms within this enthalpic framework, a "fictitious enthalpy" concept was introduced. Validation of the solver capability to handle multi-pass HEXs was done by comparing with results from analytical solutions for two-to-four pass, cross-parallel flow and cross-counter flow arrangements, showing agreement within 5%. The fully dry and fully wet solvers were validated against experimental data taken from literature for a single-pass wetted HEX, with predictions of heat transfer showing very good agreement. A parametric study was conducted on the inlet fluid temperatures and number of passes, demonstrating the substantial performance enhancement attainable with wet HEXs compared to dry counterparts. The semi-wet solver, integrating fully wet and fully dry HEX solvers, was used to identify dryout regions within the HEX under uniform and non-uniform spray inlet profiles. The proposed solver provides a versatile tool that can be coupled with results from experimental studies or CFD simulations as inputs to accurately predict the performance of spray cooled HEXs.