Micro-Heat Exchanger

With phase change particles

Universidad de Oklahoma

SUMMARY

This study shows the performance of a micro heat exchanger, for different geometric configurations and characteristics of the conducting liquid. As a thermal improvement of the base properties of water, phase change encapsulated microparticles (MEPCM) are added. These particles, by changing phase during heat exchange, absorb a greater amount of energy thanks to latent heat.

For the analysis, the Navier-Stokes equations for laminar flow have been solved. Together, as a model for the phase change process, “The temperature transforming model (TTM)” has been added to the finite volume calculation program.

The geometry and configuration of the channels, for different mass flows, have been studied to obtain the best temperature distribution throughout the heat exchanger. Parameters such as the pressure drop inside the channels and the maximum temperature value at the base have been calculated.

Once the best channel configuration has been obtained, the properties of the double-phase flow have been applied to measure the total improvement in thermal terms for the different concentrations of microparticles.

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PHYSICAL MODEL

The complete heat exchanger model is composed of 33 channels. The design has been proposed to be able to alternate the flow direction for each pair of channels. So both parallel and opposite flow can be imposed.

In addition, as an object of study, straight and zigzag-shaped channels have been proposed. The overall dimensions of the exchanger are 10 x 10 x 1 mm; being the passage section of each channel 0.2 x 0.6 mm.

RESULTS

PARALLEL VS OPPOSITE FLOW

Considering the minimum value of thermal resistance, the opposed flow configuration offers a slightly better overall thermal behavior; in addition to improving the homogeneity of temperatures at the base of the exchanger. However, the parallel flow configuration offers better results in cooling the system to lower minimum temperatures, for most of the exchanger. This configuration, therefore, offers better thermal behavior along the channels; although there is a linear temperature gradient at the base.

PARALLEL VS OPPOSITE FLOW

Considering the minimum value of thermal resistance, the opposed flow configuration offers a slightly better overall thermal behavior; in addition to improving the homogeneity of temperatures at the base of the exchanger. However, the parallel flow configuration offers better results in cooling the system to lower minimum temperatures, for most of the exchanger. This configuration, therefore, offers better thermal behavior along the channels; although there is a linear temperature gradient at the base.

STRAIGHT VS ZIGZAG CHANNELS

The zigzag channel configuration offers better thermal performance, cooling the base of the exchanger to values ​​below those achieved with the straight channel configuration. However, taking into account the relationship between thermal resistance achieved and pump power required, the discrepancies in performance disappear. The improvement in thermal performance comes at a high cost in pressure drop values.

Only in those cases in which high dissipation values ​​are required, and the pumping capacity is not a limitation, the zigzag configuration turns out to be the most suitable. After all, for low flow rates, straight channels offer good heat dissipation performance for minimal pressure drops.

FLUID WITH PHASE CHANGE PARTICLES

Finally, the calculations with double-phase fluid (water and MEPCM particles) at different concentrations are compared with those obtained for the case of parallel flow and straight channels. The results obtained show a clear improvement in the thermal dissipation properties of the fluid for any value of flow. The increase in particle concentration leads to a thermal improvement, although pressure drops also increase.

Regarding the relationship between thermal resistance values ​​and the necessary pumping power, the addition of MEPCM particles implies a thermal improvement for all cases, even reaching values ​​that the single-phase fluid was not able to reach. This implies greater thermal efficiency, achieving higher dissipation values ​​at a lower energy cost.

Finally, highlight the fact that the addition of phase change particles offers, both to the liquid and to the heat exchanger, a greater homogeneity in the distribution of temperatures along the channels. Depending on the heat power to be dissipated and the concentration of particles, it is possible to obtain the optimum flow rate, in terms of thermal efficiency, through a homogeneous distribution of phase change.

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