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The development of newer and more efficient cooling techniques to sustain the increasing power density of high-performance computing systems is becoming one of the major challenges in the development of microelectronics. In this framework, two-phase cooling is a promising solution for dissipating the greater amount of generated heat. In the present study, an experimental investigation of two-phase flow boiling in a micro-pin fin evaporator is performed. The micro-evaporator has a heated area of 1 cm2 containing 66 rows of cylindrical in-line micro-pin fins with diameter, height, and pitch of, respectively, 50 μm, 100 μm, and 91.7 μm. The working fluid is R1234ze(E) tested over a wide range of conditions: mass fluxes varying from 750 kg/m2 s to 1750 kg/m2 s and heat fluxes ranging from 20 W/cm2 to 44 W/cm2. The effects of saturation temperature on the heat transfer are investigated by testing three different outlet saturation temperatures: 25 °C, 30 °C, and 35 °C. In order to assess the thermal-hydraulic performance of the current heat sink, the total pressure drops are directly measured, while local values of heat transfer coefficient are evaluated by coupling high-speed flow visualization with infrared temperature measurements. According to the experimental results, the mass flux has the most significant impact on the heat transfer coefficient while heat flux is a less influential parameter. The vapor quality varies in a range between 0 and 0.45. The heat transfer coefficient in the subcooled region reaches a maximum value of about 12 kW/m2 K, whilst in two-phase flow it goes up to 30 kW/m2 K. [ABSTRACT FROM AUTHOR]

Multi-microchannel evaporators are often used to cool down electronic devices subjected to continuous heat load variations. However, so far, rare studies have addressed the transient flow boiling local heat transfer data occurring in such applications. The present paper introduces and compares two different data reduction methods for transient flow boiling data in a multi-microchannel evaporator. A transient test of heat disturbance from 20 to 30 W cm-2 was conducted in a multi-microchannel evaporator using R236fa as the test fluid. The test section was 1 × 1 cm2 in size and had 67 channels, each having a cross-sectional area of 100 × 100 μm2. The micro-evaporator backside temperature was obtained with a fine-resolution infrared (IR) camera. The first data reduction method (referred to three-dimensional (3D)-TDMA) consists in solving a transient 3D inverse heat conduction problem by using a tridiagonal matrix algorithm (TDMA), a Newton-Raphson iteration, and a local energy balance method. The second method (referred to two-dimensional (2D)-controlled) considers only 2D conduction in the substrate of the micro-evaporator and solves at each time step the well-posed 2D conduction problem using a semi-implicit solver. It is shown that the first method is more accurate, while the second one reduces significantly the computational time but led to an approximated solution. This is mainly due to the 2D assumption used in the second method without considering heat conduction in the widthwise direction of the micro-evaporator. [ABSTRACT FROM AUTHOR]

The pressure effects of expanding the cross section of microchannels along the direction of flow are investigated across four rates of channel expansion in the flow boiling of R-134a. Prior investigation by the authors detailed the fabrication of four copper micro-channel arrays and the pumped-loop apparatus developed to facilitate interchange of the microchannel specimens, allowing consistency across experiments. Significant beneficial pressure effects are observed to result from the expansion, including reduction by half of the pumping cost per flow rate at critical heat flux. The improvements are seen with small expansions, and greater expansion yields diminishing returns. The high pressure drops associated with microchannel evaporators are effectively reduced by expanding channel geometry, and the low-frequency system spectral response indicates that expanding chan-nel arrays typically carry less energy in oscillations up to 2.5Hz, suggesting amelioration of oscillatory instabilities. Results are discussed in light of a comparative force analysis, with the balance of these forces linked to the observed behavior of the pressure drop and heat flux relationship. [ABSTRACT FROM AUTHOR]

Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Diyout is generally considered a failure of the heat pipe operation. However, traditional diyout theoiy may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupttion of liquid transport passages. This article investigates the diyout phenomenon in carbon nanotube (CNT) based hiwick structure. The incipience and expansion of the dryout zone on the CNT hiwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the hiwick diyout. Results based on both visualization and measurement show that diyout of CNT hiwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local diy zone. On the curves of heat flux versus the evaporator temperature, diyout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on hiwick structures when compared with traditional monowick structures. [ABSTRACT FROM AUTHOR]

Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance the boiling heat transfer mechanism in the refrigerant channel. Various distance frames in the refrigerant channel were also employed to study the influence of the refrigerant mass flux on two-phase flow heat transfer. The tests were conducted at heat fluxes ranging between 4.5 kW/m2 and 17 kW/m2 with 134a as refrigerant. Pool boiling tests of the enhancement structure, under similar conditions and at various surface inclination angles, were also performed for reasons of comparison. The plate heat exchanger with the enhancement structure displayed up to ten times enhanced heat transfer coefficient in the refrigerant channel, resulting in an improvement in the overall heat transfer coefficient with over 100%. This significant boiling enhancement is in agreement with previous pool boiling experiments and confirms that the enhancement structure may be used to enhance the performance of plate heat exchangers. A simple superposition model was used to evaluate the results, and it was found that, primarily, the convective boiling mechanism was affected by the distance frames in the standard heat exchanger. On the other hand, with the enhanced boiling structure, variations in hydraulic diameter in the refrigerant channel caused a significant change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance. [ABSTRACT FROM AUTHOR]

Miniature loop heat pipes (mLHPs) are coming up with a promising solution for the thermal management of futuristic electronics systems. In order to implement these devices inside compact electronics, their evaporator has to be developed with small thickness while preserving the unique thermal characteristics and physical concept of the loop scheme. This paper specifically addresses the design and testing of a mLHP with a flat evaporator only 5 mm thick for the cooling of high performance microprocessors for electronic devices. A novel concept was used to achieve very small thickness for the mLHP evaporator in which the compensation chamber was positioned on the sides of the wick structure and incorporated in the same plane as the evaporator. This is unlike the conventional design of the flat evaporator for mLHP in which the compensation chamber, as a rule, adds to the overall thickness of the evaporator. The loop was made from copper with water as the heat transfer fluid. For capillary pumping of the working fluid around the loop, a sintered nickel wick with 3-5 µm pore radius and 75% porosity was used. In the horizontal orientation, the device was able to transfer heat fluxes of 50 W/cm² at a distance of up to 150 mm by using a transport line with 2 mm internal diameter. In the range of applied power, the evaporator was able to achieve steady state without any temperature overshoots or symptoms of capillary structure dryouts. For the evaporator and condenser at the same level and under forced air cooling, the minimum value of 0.62°C/W for mLHP thermal resistance from evaporator to condenser (Rec) was achieved at a maximum heat load of 50 W with the corresponding junction temperature of 98.5°C. The total thermal resistance (Rt) of the mLHP was within 1.5-5.23°C/W. At low heat loads, the mLHP showed some thermal and hydraulic oscillations in the transport lines, which were predominately due to the flow instabilities imposed by parasitic heat leaks to the compensation chamber. It is concluded form the outcomes of the present investigation that the proposed design of the mLHP evaporator can be effectively used for the thermal control of the compact electronic devices with high heat flux capabilities. [ABSTRACT FROM AUTHOR]