Heat Transfer and Pressure Drop in Flow Boiling in Microchannels

Microchannels are being appreciated and observing lot of Research and development work associated with their application in every discipline of engineering. The fortes of microchannels are (1) ability to provide high area to volume ratio so that high heat flux may be readily dissipated and (2) potential to meet compact and low weight design criterion. Though the need of reducing the size of heat exchangers was already prevailing but reducing size to micro level is essentially due to advances in electronic industry. Development of high speed processors, inclusion of multi tasking features in electronic devices like mobile and computers have lead to very large scale integrated circuits on a very small size board and the large integration of components in a very small area posed a problem of electronic cooling. To combat this situation, Tuckerman and Pease [1] gave the clue of using very small size channels for dissipating high heat flux. With this pioneer demonstration, many research works pertaining to microchannels fluid flow and heat transfer characteristics are being reported globally.

In order to comprehend the heat transfer and pressure drop characteristics of two phase flow through mini-microchannels, identification of different flow regimes prevailing in two phase flow is considered to be the first and most important step towards the analysis and this is being done using high speed flow visualization techniques. The main objective of the flow visualization study is to provide insight into flow distribution, flow regimes, and transition characteristics in microchannels. The thermohydraulic performance of these reduced size channels are strongly influenced by the number of flow regimes encountered. All those factors that can affect the flow regimes need to be identified for improved design of microchannel heat sinks. The summary of flow patterns recorded during flow visualization is presented in the Table 2.1 below.

Though two phase flow offers advantage of dissipating high heat flux at reduced mass flow rate while maintaining constant coolant temperature, a problem of higher pressure drop compared to the single phase flow is accompanied in two phase flow. Hence this issue needs to be resolved in order to develop an economically viable heat sinks. Efforts are being continued to identify the sources responsible for large pressure drop. In this section studies (experimental/ analytical) on pressure drop in microchannels are presented.

Microchannels are emerging most promising technology for thermal management in electronic devices or wherever high heat flux is to be dissipated from very small size surface area. Among various cooling methods, two phase cooling method outperform. Flow boiling can dissipate significantly higher heat fluxes while requiring smaller rates of coolant flow than its single-phase counterpart moreover heat exchanger surfaces are subjected to more uniform temperature which in turn provides longevity to the components. Despite these appealing adjectives, their applications in practical domain are limited. Two phase flow through microchannels experience larger pressure drop than single phase flow and is the major concern. In an effort to address this issue, flow visualization has been utilized by many researchers and this technique has greatly helped in understanding the two phase flow mechanism. Through flow visualization different flow patterns and different flow regimes have been identified. The change of flow regime from one to another has been one of the potential causes affecting pressure drop and heat transfer characteristics in two phase flow. Pressure and thermal oscillations are the major concern. Since thermal performance of microchannels are drastically reduced owing to dry out conditions prevailing due to flow instability. Some prominent techniques advocated to suppress the instability of flow boiling in microchannels are: inlet constriction, diverging channel, artificial nucleation sites and seed bubble technique. The success of two phase cooling technology is essentially dependent upon the development of measuring techniques. The more accurately measurement is done; the more accurate predictions regarding the thermohydraulic performance of flow boiling can be done.