Automobile radiators

The work carried out in CTTC on this applied topic has been the core of our fin-and-tube heat exchangers applied research line. As a consequence of several years of academic research and industrial collaboration, an interesting combination of a distributed numerical model, experimental studies and CFD analysis has been developed.

Distributed numerical model

The Group started the development of a distributed numerical model to analyze compact heat exchangers (called CHESS) in order to study automobile radiators. Their geometry is mainly a flat tube array brazed with corrugated louvered fins between them. The internal flow is managed by inlet/outlet manifolds that divide the coolant within several passes. The code has also been adapted to other automobile heat exchangers, such as charge-air coolers or oil coolers, with the corresponding set of particular geometries (internal fins, offset-strip inserts, etc.).

The numerical method is based on the discretization of the heat exchanger in a set of macro control volumes around the tubes, where governing equations are applied to obtain the 3D temperature, velocity and pressure maps.

Fig 1-2.: Discretization of the heat exchanger core

The corresponding fins are modeled by using analytical fin efficiency model between two adjacent tubes, thus calculating the heat transfer at the control volume boundary (inter-tube thermal bridge). For the coolant side, a 1D model is applied, where the empirical heat transfer and friction coefficients have been implemented considering tube aspect ratio and flow development. An additional module based on a pressure-correction scheme has been developed in order to determine the flow distribution within the radiator (multi-pass arrangement, bypassed manifolds, etc.).

Fig 3-4.: Multi-pass flow arrangement and corresponding distribution mesh

The model provides at each time step, a detailed distribution of flow, pressure and temperature for air, coolant and solid elements throughout the radiator. The results are typically generated for a matrix of air and coolant flows, obtaining a performance map that can be used to compare several alternatives and to find improved designs.

Fig.5-6: Heat transfer performance maps for different fin pitches

Experimental set-up

The experimental facility for gas-liquid compact heat exchangers testing consists of a climatic chamber (where the test prototype is installed), a compensating/regulating chamber, and a coolant loop. The objective of this set-up is to test full-scale heat exchangers at desired operating conditions, and obviously to quantify the heat transfer rate and the air/coolant pressure drops at those conditions.
The air circuit of the test facility assures stable controllable conditions of the air flow at the inlet test section. The air is sucked from the climatic chamber through the test section and is blown through the pipe to the compensating-regulating chamber, where heat is added to the air in order to maintain the inlet temperature at desired conditions. The air flow rate can vary up to 7500 m3/h by means of a variable speed centrifugal fan.
The liquid circuit has been designed to assure desired stable liquid inlet conditions at the tested radiator, allowing the analysis of different coolants as well as the free adjustment of liquid flow rate (up to 5000 kg/h).

Fig. 7: Experimental air loop to test fin-and-tube heat exchangers.
Fig. 8: Radiator sample and associated instrumentation and ducting.

A set of experimental data is usually collected by varying the air and refrigerant flows, obtaining the corresponding performance curves for heat transfer and pressure drop. The results are then compared against the numerical predictions for validation purposes.

Fig. 9 -10: Experimental-numerical comparison for a motorbike radiator (heat transfer and coolant pressure drop)

CFD analysis through fin-and-tube core

The distributed model has been usually fed by open-literature heat transfer and friction correlations in order to predict accurately and without any experimentally based correction the numerical results. However, this approach is limited to available geometries and ranges. Considering the excellent CFD background of the research Group, in-house CFD studies on the fin-and-tube geometries involved in automobile radiators are being carried out. After proper validation against published and own experimental results, the combination of CFD-based correlations with a very detailed distributed model results in a very powerful design tool.

 
Fig. 11-12: CFD simulation of the airflow path through a louvered fin-and-tube passage: domain and 3D temperature maps.