a b s t r a c tMicrochannel heat sinks using two-phase flow boiling have excellent potential for cooling high heat flux electronic devices. A numerical model of a two-phase cooling system using microchannel heat sinks is presented. This is supplemented with transient heat conduction models of the heat sink and heat exchanger and with various state of the art empirical correlations to close the equations. Finite volume discretization and the SIMPLE algorithm are used to solve the mixture conservation equations of mass, momentum and energy. The numerical code is verified using the method of manufactured solutions. This method reveals that the numerical order in space and time is consistent with the expected values from theory, second order and first order, respectively. Further some illustrative results of the model are validated using experimental measurements. These results agree well with each other and indicate good predictive capability of the system model.

Heat sinks with liquid forced convection in microchannels are targeted for cooling electronic devices with a high dissipated power density. Given the inherent stability problems associated with two-phase microchannel heat transfer, this paper investigates experimentally the potential for enhancing single-phase convection cooling rates by applying pulsating flow. To this end, a pulsator device is developed which allows independent continuous control of pulsation amplitude and frequency. For a single minichannel geometry (1.9 mm hydraulic diameter) and a wide range of parameters (steady and pulsating Reynolds number, Womersley number), experimental results are presented for the overall heat transfer enhancement compared to the steady flow case. Enhancement factors up to 40% are observed for the investigated parameter range (Reynolds number between 100 and 650, ratio of pulsating to steady Reynolds number between 0.002 and 3, Womersley number between 6 and 17). Two regimes can be discerned: for low pulsation amplitude (corresponding to a ratio of pulsating to steady Reynolds number below 0.2), a small heat transfer reduction is observed similar to earlier analytical and numerical predictions. For higher amplitudes, a significant heat transfer enhancement is observed with a good correspondence to a power law correlation. This work establishes a reference case for future studies of the effect of flow unsteadiness in small scale heat sinks.

A one dimensional dynamic system model is developed to accurately simulate a two-phase microchannel electronics cooling loop. This model is based on the single component mixture equations for mass, momentum and energy. These equations are solved numerically using a finite volume method in conjunction with the SIMPLE algorithm. To calculate the pressure losses and heat transfer state of the art empirical correlations are used. Furthermore size effects of a typical microchannel cooling system are investigated with the new model. Special attention is given to the accumulator size and its limitations for portable applications. A simple model to investigate the accumulator size effect on the loop is developed and compared to numerical results obtained from the system model. The influence of various loop parameters and possible improvements are also investigated. Finally the effect of using different coolants is studied.

Using a state of the art one dimensional, two-phase, dynamic system model, the size effects of a portable two-phase microchannel electronic cooling system are investigated using different refrigerants (R134a, R236fa, R245fa). Special attention is given to the accumulator size and its limitations for portable applications. An analytical model is developed to investigate the accumulator size effect on the loop and compared to the numerical results obtained from the system model. The influence of various loop parameters and possible improvements is investigated. Finally usable design equations are developed to calculate the needed accumulator size. These equations are further used to compare the performance of the different refrigerants and control techniques with regard to the size effects.

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