The efficient heat transfer resulting from flow boiling in microchannel heat sinks can help dissipate high heat fluxes from high-density electronic devices across a small temperature difference. However, practical implementation challenges unique to two-phase flow boiling, as compared to single-phase liquid cooling, have prevented its widespread adoption. A primary challenge is the occurrence of dynamic twophase flow instabilities, such as pressure drop oscillations (PDO), that have the potential to degrade heat transfer performance or trigger premature critical heat flux (CHF) under some conditions. Under other conditions, PDOs are observed to have little to no impact on performance. One factor proposed by modeling studies to be responsible for this discrepancy in observations of the effect of dynamic instabilities on performance is the thermal capacitance of the heat sink, though this has not been confirmed by experiments. In this study, the effect of thermal capacitance on the transient thermal response of a heat sink experiencing PDOs is examined through use of a dynamic two-phase flow model and experiments. Flow boiling experiments are performed with a controlled compressible volume upstream of parallelmicrochannel heat sinks having either a large or a small thermal capacitance. In accordance with the behavior predicted by our model, when thermal capacitance is reduced, the pressure drop oscillation frequency is found to decrease and temperature swings in the heat sink become more severe. Additionally, the experimentally measured CHF limit is diminished in the heat sink at smaller thermal capacitance. These results reveal thermal capacitance as a critical parameter that determines how much dynamic instabilities degrade flow boiling performance in a microchannel heat sink. & COPY; 2023 Elsevier Ltd. All rights reserved.

Microchannel flow boiling heat sinks that leverage the highly efficient heat transfer mechanisms associated with phase change are a primary candidate for cooling next-generation electronics in electric vehicles. In order to design flow boiling heat sinks for such practical applications, one key obstacle is an understanding of the conditions for occurrence of dynamic two-phase flow instabilities, to which microscale flow boiling is particularly susceptible, as well as their impact on heat transfer performance. While mapping the operational regimes of these instabilities has been well-studied, with numerous stability criteria available, their impact on the heat transfer performance of heat sinks in practical applications is not understood. This work seeks to assess the impact of pressure drop oscillations and parallel channel instabilities on the surface temperature and critical heat flux in parallel microchannel heat sinks. This is achieved through measurement of time-averaged steady-state temperatures and pressures, combined with high-frequency pressure signals and high-speed flow visualization. These data are compared across three controlled flow configurations that comprise a condition of stable flow boiling, a condition where only parallel channel instabilities can occur, and a third where both pressure drop oscillations and parallel channel instabilities can occur. Experiments are performed using the dielectric refrigerant HFE-7100 in 2 cm long parallel microchannel heat sinks with square-cross-section channels (0.25, 0.5, 0.75, and 1 mm widths) at three mass fluxes (100, 400, and 1600 kg/m2s). Across this range of conditions, the time-averaged surface temperature and critical heat flux were remarkably insensitive to the occurrence of these instabilities despite the significant hydrodynamic events and transient flow patterns observed.

Flow boiling in microchannel heat sinks is capable of providing the high-heat-flux dissipation required for thermal management of next-generation wide bandgap power electronics at low pumping power and uniform surface temperatures. One of the primary issues preventing implementation of these technologies is the presence of flow boiling instabilities, which may reduce the heat transfer performance. However, the effect of individual instabilities, such as the parallel channel instability or pressure drop oscillations, on the overall heat transfer coefficient and critical heat flux in microchannel heat sinks has not been fully quantified. The primary cause of these dynamic flow boiling instabilities is the interaction between the inertia of a two-phase mixture in a heated channel and sources of compressibility located upstream of the inlet. In order to isolate the effect of pressure drop oscillations on flow boiling heat transfer performance, experiments are performed in a single-square microchannel cut into a copper heat sink, with a controlled level of upstream compressibility. The impact of pressure drop oscillations on the heat transfer coefficient and critical heat flux is characterized through analysis of both time-averaged steady-state data as well as high-frequency pressure signals synchronized with high-speed visualization. The dielectric working fluid HFE-7100 is used in all experiments with a saturation temperature of 60 °C at the channel outlet pressure. The occurrence and effect of pressure drop oscillations in 20-mm-long microchannels of three different channel widths (0.5, 0.75, and 1 mm) are related to mass flux, the degree of two-phase flow confinement, and the severity of pressure drop oscillations.

Thermal management of high-power electronics requires cooling strategies capable of dissipating high heat fluxes while maintaining the device at low operating temperatures. Two-phase jet impingement offers a compact cooling technology capable of meeting these requirements at a low pressure drop. Generally, confined impingement geometries are used in electronics cooling applications, where the flow is constrained between the hot surface and orifice plate. Understanding the primary heat transfer mechanisms occurring as boiling takes place on the surface during jet impingement is important, specifically under such confined conditions. In this study, heat transfer from a copper surface is experimentally characterized in both confined jet impingement and pool boiling configurations. The dielectric liquid HFE7100 is used as the working fluid. For the jet impingement configuration, the jet issues through a single 2 mm-diameter orifice, at jet exit velocities of 1, 3, 6, and 9 m/s, into a confinement gap with a spacing of 3 jet diameters between the orifice and heat source. Additional orifice-to-target spacings of 0.5, 1, and 10 jet diameters are tested at the lowest (V-j =1 m/s) and highest (V-j = 9 m/s) jet velocities. By incrementing the heat flux applied to the surface and observing the steady-state response at each flux, the single-phase and two-phase heat transfer performance is characterized; all experiments were carried through to critical heat flux conditions. The jet impingement data in the fully boiling regime either directly overlap the pool boiling data, or coincide with an extension of the trend in pool boiling data beyond the pool boiling critical heat flux limit. This result confirms that nucleate boiling is the dominant heat transfer mechanism in the fully boiling regime in confined jet impingement; the convective effects of the jet play a negligible role over the wide range of parameters considered here. While the presence of the jet does not enhance the boiling heat transfer coefficient, the jet does greatly increase single-phase heat transfer performance and extends the critical heat flux limit. Critical heat flux displays a linear dependence on jet velocity while remaining insensitive to changes in the orifice-to-target spacing. (C) 2018 Elsevier Ltd. All rights reserved.

Two-phase jet impingement is a compact cooling technology capable of dissipating the large heat fluxes required for thermal management of high-power electronics devices. It is important to understand the primary heat transfer mechanisms that occur during regimes of jet impingement for which boiling occurs, specifically in the confined impingement geometries common to electronics cooling applications. In this study, heat transfer from a surface is experimentally characterized in both confined jet impingement and pool boiling configurations. The dielectric liquid HFE-7100 is used as the working fluid. For the impingement configuration, the jet issues through a single orifice with a diameter of 2 mm, at exit velocities of 1 m/s and 3.33 m/s, into a confinement gap with an orifice-to-target spacing ratio of 3. Additional orifice-to-target spacings of 0.5 and 5 times the jet diameter are tested at the lower jet velocity. The heat flux applied at the surface was increased in steps to characterize the single-phase and two-phase heat transfer performance; all experiments were carried through to critical heat flux conditions. Over the range of velocities and orifice-to-target spacings tested, the jet impingement data in the fully boiling regime coincide with the pool boiling data. This result indicates, for the range of parameters considered in this study, that nucleate boiling is the dominant heat transfer mechanism in the fully boiling regime in confined jet impingement. The impinging jet velocity and orifice-to-target spacing only influence the single-phase heat transfer and critical heat flux.

Two-phase jet impingement is a compact cooling approach that provides high-heat-flux dissipation at manageable pressure drop. The heat transfer behavior of an impinging jet array is dependent on a set of geometrical parameters, operating conditions, and fluid properties. In the present study, a semi-empirical approach is developed to predict heat transfer from arrays of jets of liquid that undergoes phase change upon impingement. In the model, the jet array is divided into unit cells centered on each orifice that are assumed to behave identically. The impingement surface in each unit cell is divided into two distinctive regions: a single-phase heat transfer region directly under the jet, and a surrounding boiling heat transfer region along the periphery. Available correlations from the literature are used to estimate the heat transfer coefficient and surface temperature distribution in each region, and the mean surface temperature of the unit cell is estimated via area-averaging. The location of transition to boiling predicted by the model is consistent with prior experimental observations of an inward-creeping boiling front. The model results are first compared against existing experimental data in the literature, and the area-averaged thermal performance is found to be well-predicted. Additional experiments are also performed to evaluate the limits of applicability of the model. The semi-empirical modeling approach developed in this work successfully represents the different heat transfer modes and transitions that occur during two-phase jet impingement.