Research Interests in Thermal/Fluid Sciences
Research background:
Other areas of interest:
- Electronics cooling/thermal management
- Micro-fluidic/thermal devices
Thermal/Fluid Team: Here.
Description of the past and current research activities:
Micro-channels two-phase flow
Two-phase flow analysis for the evaporation and condensation of refrigerants within the micro-channels is an area of ongoing research. The two-phase flow characteristics in micro-channels may be more sophisticated than conventional macro-channels, and the theories and empirical correlations for one scale may not work for the other one. The objective of this research topic is to investigate the parameters that affect the two-phase heat transfer within the micro-channels, and to utilize the dimensional analysis technique to develop appropriate correlations for specific geometries.
An application of this research study is the heat transfer and fluid flow of refrigerants in brazed plate heat exchangers, known as BPHEs. Thermo-hydrodynamic performance of few BPHEs has been analyzed experimentally, and the relevant results have been presented in Jokar et al. [2004, 2005, & 2006]. These heat exchangers were used as the evaporator and condenser of an automotive refrigeration system where the refrigerant R-134a flowed on one side and a 50% glycol-water mixture on the other side in a counter-flow configuration. The heat transfer coefficient for the single-phase flow of the glycol-water mixture was first obtained using a modified Wilson plot technique. The results from the single-phase flow analysis were then used in the two-phase flow analysis, and correlations for the refrigerant evaporation and condensation heat transfer were developed. Correlations for the single-phase and two-phase Fanning friction factors were also obtained based on a homogenous model. The results of these studies have shown that the two-phase theories and correlations that were established for conventional macro-channels may not hold for the micro-channels.
Figure 1.
Cutaways of typical brazed plate heat exchanger .
Mini-channels compact heat exchangers
The single-phase flow analysis of the mini-channel air-liquid heat exchangers has continuously been under investigation. Many reports agreed that the transport phenomena of the mini-size heat exchangers are different from the classical theories which have been established for the conventional macro-size heat exchangers. However, several experimental researches observed this difference only for the heat transfer correlations, while the pressure drop can still be estimated as the same.
An example of this research study is the heat transfer and fluid flow of cooling fluids in mini-channel compact heat exchangers. For this purpose, flow of glycol-water mixture in one side and air on the other side of few automotive heat exchangers have experimentally been analyzed, and the results have been presented in Jokar et al. [2004 & 2005] and compared to the other studies. The well-known Wilson plot technique was applied to obtain the heat transfer coefficients, and the Fanning equation was used to calculate the pressure drop friction factors. The uncertainty estimates for the measured and calculated parameters were also presented. The results of this study agreed that the heat transfer and pressure drop correlations developed for the macro-size channels cannot accurately be applied for the mini-channels of the small-size heat exchangers.
Figure 2. Cutaways of typical mini-channel compact heat exchangers.
Automotive thermal systems integration
Most cars use separate cooling and heating systems to control the cabin interior air temperature for passenger comfort. The cooling system uses a standard refrigeration loop to cool the cabin in summer while the heating system uses the heat supplied from the engine to warm the cabin in winter. However, in recent automotive applications with alternative fuel sources, such as fuel cell or direct-injection diesel engines, the heat rejected from the engine is not sufficient to warm the cabin in cold climates. On the other hand, the cooling system which normally uses a vapor compression refrigeration cycle is able to cool the cabin as desired shortly after starting the engine in warm climates. A dual-loop cooling and heating system, which can work in both cooling and heating modes, may overcome the delayed heating problem and supply sufficient thermal energy to the passenger cabin in cold seasons.
For this purpose, the integration of an automotive thermal system has been studied, and the results presented in Jokar et al. [2004 & 2005]. The integrated system was able to cool or heat the passenger cabin of a conventional automobile. The proposed system integrated both air conditioning and heat pump modes so that, by simply switching a series of valves, the cabin could be cooled or heated. In this research project, such a dual-loop automotive thermal system was designed and fabricated. Experiments were conducted and experimental data were collected and analyzed. The test results showed that the dual-loop system was a viable option resulting in acceptable coefficients of performance, as compared to the conventional automotive air conditioning system. Further optimization of the proposed integrated system is expected to enhance performance of the system.
Figure 3.
The dual-loop automotive thermal system designed, fabricated, and tested in the IER at K-State.
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