水平窄空间沸腾传热的实验研究

通过对五种尺寸的窄空间试验元件分别以水和乙醇做工质进行实验。研究了窄空间间距、窄空间尺寸、不同工质及不同热流密度对窄空间沸腾性能的影响。结果表明：当窄空间尺寸与热流通等因素组合恰当时。其换热系数可比大空间池沸腾提高3～6倍；临界热流密度有所降低。     

Efficient spreaders for cooling high-power computer chips

Investigated is the performance of composite spreaders, consisting of a top layer of porous graphite (⩾0.4 mm), for enhanced cooling by nucleate boiling of FC-72 dielectric liquid, and a copper substrate (⩽1.6 mm), for efficient spreading of the dissipated thermal power by the underling 10 × 10 mm or 15 × 15 mm high-power computer chips. The analysis assumes uniform thermal power dissipation by the chips and calculates the square surface area of the spreader, along with the spreading, boiling and total thermal resistances, the maximum chip temperature, and the removed thermal power from the spreader surface by saturation or subcooled nucleate boiling of FC-72 liquid. These performance parameters are determined as functions of the thickness of the copper substrate and the size of the underlying chip. When compared with those of copper and porous graphite spreaders of the same total thickness, 2.0 mm, the performance of the composite spreaders is superior for cooling high-power computer chips. When cooled by nucleate boiling of 30 K subcooled FC-72 liquid, the composite spreader removes 160.3 W and 98.4 W of dissipated thermal power by the underlying 10 × 10 mm and 15 × 15 mm chips, at total thermal resistances of 0.29 and 0.48 °C/W. When the same spreader is cooled by saturation boiling of FC-72, the removed thermal power decreases to 85.6 W and 53.4 W, and the total thermal resistances also decrease to 0.12 and 0.20 °C/W, respectively. Although the calculated surface temperatures of the chips are not uniform, the maximum temperatures are <71 °C and the temperature differential across the chips is <8 °C. For the same cooling condition, the calculated surface area of the copper spreaders, the total thermal resistance, and the maximum chip temperature are much higher, but the removed thermal powers from the surface of spreaders are much lower than with composite spreaders. The calculated surface areas of the porous graphite spreaders are smaller, the thermal powers removed from surface of these spreaders are much lower and both the total thermal resistance and the maximum chip temperature are higher than those with composite spreaders.     

Experimental research of pool boiling heat transfer in horizontal narrow spaces

Much progress has been made in high‐performance electronic chips, the miniaturization of electronic circuits and other compact systems recently, which brings about a great demand for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, experiments were carried out on five kinds of test elements with distilled water and ethanol as working liquids. The test elements used in these experiments consisted of five parallel discs with diameters varying from 5 mm to 40 mm. The experiments were performed with the discs oriented horizontally and uniform heat fluxes applied at the bottom surfaces. The influence of narrow spacing, space size, working liquid property, and heat flux on boiling heat transfer performance in narrow spaces has been investigated. Experimental results showed that the boiling heat transfer coefficient of a narrow space was 3 to 6 times higher than that of pool boiling when the narrow space size and heat flux combine adequately, but the critical heat flux was lower than that of pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 307–315, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20017     

Subcooled flow boiling heat transfer of FC-72 from silicon chips fabricated with micro-pin-fins

Experiments were conducted to study the subcooled flow boiling heat transfer performance of FC-72 over silicon chips. For boiling heat transfer enhancement, two kinds of micro-pin-fins having fin thickness of 50 μm and fin heights of 60 and 120 μm, respectively, were fabricated on the silicon chip surface with the dry etching technique. The fin pitch was twice the fin thickness. The experiments were conducted at the fluid velocities of 0.5, 1 and 2 m/s and the liquid subcoolings of 15, 25 and 35 K. The micro-pin-finned surfaces showed a sharp increase in heat flux with increasing wall superheat and a large heat transfer enhancement compared to a smooth surface. The nucleate flow boiling curves for the two micro-pin-finned surfaces collapsed to one line showing insensitivity to fluid velocity and subcooling, while the critical heat flux values increased with fluid velocity and subcooling. The micro-pin-finned surface with a larger fin height of 120 μm provided a better flow boiling heat transfer performance and a maximum critical heat flux of 145 W/cm². The wall temperature at the critical heat flux for the micro-pin-finned surfaces was less than 85 °C for the reliable operation of LSI chips.     

Two-Phase Heat Spreaders Utilizing Microfabricated Boiling Enhancement Structures

Results from various enhancement techniques to improve the performance of a recently developed two-phase heat spreader are reported. The spreader has a central evaporator section, with an integrated condenser along the edges. A micro-fabricated three-dimensional enhancement structure is employed to improve the heat transfer performance of the spreader. This study considers the performance of several liquid coolants to be used with the device and evaluates the effect of initial pressures on the thermal performance of the spreader plate. Liquids with lower boiling points were found to result in lower wall temperatures of the spreader plate due to earlier onset of boiling. Studies also showed an improvement in heat transfer performance with increase in stack height of the enhancement structures used in the evaporator section.     

常压下狭缝流动沸腾传热的研究

以蒸馏水为工质，在常压下，对间隙为1mm的环形狭缝通道中的流动传热进行了实验研究。分别将狭缝通道中的单相强制对流和过冷沸腾的实验数据与传统的Dittus-Boelter型关系式的计算结果进行了比较。通过分析狭缝通道中流动沸腾的传热特性认为，过冷沸腾传热比单相强制对流传热加强；质量流速对狭缝通道中的流动沸腾传热有较大影响。     

Development of Phase Change Heat Spreader for Treatment of Intractable Neocortical Epilepsy

A combined analytical and experimental investigation was conducted to develop a flat, phase change heat spreader to enable focal thermoelectric cooling as a treatment for intractable neocortical epilepsy. The design parameters required minimum transport capacity of 5 W with an associated temperature drop of less than 0.5°C. A one-dimensional steady-state model was developed and the predicted performance characteristics compared with the results obtained from the experimental evaluation of three conceptual designs of varying complexity. These designs varied in terms of the materials used, but all three used water as the working fluid. The experimental results indicate that the minimum transport limit of 5 W can be achieved by one of the three concepts evaluated with a maximum overall temperature drop of 0.36°C at 5 W, well below the 0.5°C limit and within the experimental uncertainty of the temperature measurement technique used. A simple model was used to aid in the selection of an appropriate heat pipe. Using the verified model, the initial design was optimized, and based upon this optimized design, a number of test articles were fabricated and evaluated experimentally.     

Experimental investigation of small diameter two-phase closed thermosyphons charged with water,FC-84, FC-77 and FC-3283

An experimental investigation of the performance of thermosyphons charged with water as well as the dielectric heat transfer liquids FC-84, FC-77 and FC-3283 has been carried out. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm, which can be considered quite small compared with the vast majority of thermosyphons reported in the open literature. The evaporator length was 40 mm and the condenser length was 60 mm which corresponds with what might be expected in compact heat exchangers. With water as the working fluid two fluid loadings were investigated, that being 0.6 ml and 1.8 ml, corresponding to approximately half filled and overfilled evaporator section in order to ensure combined pool boiling and thin film evaporation/boiling and pool boiling only conditions, respectively. For the Fluorinert? liquids, only the higher fill volume was tested as the aim was to investigate pool boiling opposed to thin film evaporation. Generally, the water-charged thermosyphon evaporator and condenser heat transfer characteristics compared well with available predictive correlations and theories. The thermal performance of the water-charged thermosyphon also outperformed the other three working fluids in both the effective thermal resistance as well as maximum heat transport capabilities. Even so, FC-84, the lowest saturation temperature fluid tested, shows marginal improvement in the heat transfer at low operating temperatures. All of the tested Fluorinert? liquids offer the advantage of being dielectric fluids, which may be better suited for sensitive electronics cooling applications and were all found to provide adequate thermal performance up to approximately 30–50 W after which liquid entrainment compromised their performance.     

水平窄空间沸腾换热的数理模型研究

窄空间只有在间距小于汽泡脱离直径时，对沸腾传热强化才有比较显的效果。窄空间沸腾强化传热的机理在于较大的泡底微层加速了蒸发传热和窄空间中被加热的液体周期性地与池液进行容积交换。水平圆盘窄空间中的汽泡生长分为性质完全不同的自由生长期和抑制长大期；在一个周期内，加热面的总传热量等于壁面传导给窄空间液体的热量与通过合体泡底微层蒸发潜热之和。在对圆形水平窄空间的沸腾传热的现象和机理进行分析的基础上，提出了窄空间的沸腾换热过程的数理模型；进而对窄空间沸腾的本质规律在理论上进行了初步探索，并得到分析解。理论计算结果与实验数据比较表明，该分析解适合于中低壁面过热度的情形。由于问题的复杂性，该模型仍需不断完善。     

An Experimental Investigation of Thermal Performance of a Polymer‐Based Flat Heat Pipe

A polymer‐based flat heat pipe (PFHP) was fabricated. The heat transfer performance was measured and analyzed when deionized (DI) water and acetone were used as the working fluid, separately. Input power ranging from 2.8 W to 14.2 W was provided to the evaporator section while the device was at different filling ratios. Experimental results revealed that, when the polymer‐based flat heat pipe was laid in a horizontal position, the thermal resistance (1.02 K/W) was much smaller than that (4.6 K/W) of a copper plate with the same thickness at the thermal power of 10.3 W and the value decreased as the tilt angle changed from 0° to 90°.     

Single-phase and two-phase cooling using hybrid micro-channel/slot-jet module

This paper explores the single-phase and two-phase cooling performance of a hybrid micro-channel/slot-jet module using HFE-7100 as working fluid. Three-dimensional numerical simulation using the k–ε turbulent model is used to both assess the single-phase performance and seek a geometry that enhances heat removal capability and surface temperature uniformity while decreasing mean surface temperature. This geometry is then tested experimentally to validate the numerical findings and aid in the development of correlations for both the single-phase and two-phase heat transfer coefficients. The hybrid module is shown to maintain surface temperature gradients below 2 °C for heat fluxes up to 50 W/cm². Even without phase change, the hybrid module is capable of dissipating heat fluxes as high as 305.9 W/cm². Highly accurate single-phase correlations are developed using a superpositioning technique that consists of assigning a different heat transfer coefficient for each portion of the heat transfer area based on the dominant heat transfer mechanism for that portion. Increasing subcooling and/or flow rate is shown to delay the onset of nucleate boiling to a higher heat flux and higher surface temperature, as well as enhance critical heat flux (CHF). A correlation previously developed for hybrid micro-channel/micro-circular-jet module is deemed equally effective at predicting two-phase heat transfer data for the present hybrid module.     

强化传热管束狭窄空间内R_11的沸腾换热特性

对紧凑型滚压面传热管管束狭窄空间内R－11的沸腾强化换热进行了实验研究，确认了由紧凑型滚压强化管束组成的满液式蒸发换热器具有良好的换热性能。其原理是利用强化传热管管束狭窄空间提前从自然对流换热转换为旺盛核沸腾换热，实验结果确认了管束形成的狭窄空间和强化传热面两种强化技术对沸腾换热的强化效果不能简单叠加。     

Development of a beta-type Stirling heat pump with rhombic drive mechanism by a modified non-ideal adiabatic model

In this paper, a thermodynamic model is developed for predicting the performance of a beta-type Stirling heat pump with rhombic drive mechanism for water heater and the model is validated by a 1-kW class prototype Stirling heat pump. In the present model, the working space is divided into expansion space, heat absorber, regenerator, heat rejecter and compression space. The pressure, mass and temperature variations of working fluid in each working space are predicted. The temperature variation of wall boundary is also taken into consideration. The temperature of working fluid in each working space and the temperature of wall boundary are obtained by solving energy equations simultaneously. Eventually, the pressure of working fluid in each working space can be corrected by using empirical formula of pressure drop. All the thermal properties of working fluid and wall boundary in each working space at each time step can be obtained by repeating the above process. Then, the performance of heat pump such as absorbing heat, rejecting heat, indicated power and COP can be calculated. A series of experimental measurements and comparisons are also conducted for validating present model. The results show that the prototype heat pump can produce 904 W heating power and 38°C hot water under 1 LPM water flow rate with 5 bar helium at 1000 rpm.     

The effects of vibration and reciprocating on boiling heat transfer in cylindrical container

The performance of enhancing boiling heat transfer in cylindrical container was studied experimentally. The effects of the heat load, the grain size of the steel and plastic balls added in the container, the vibration frequency of the disk, and the reciprocation frequency of the cylindrical container on the characteristics of heat transfer were investigated in detail for water as working fluid. The experiment results show that the active methods can really improve the boiling heat transfer promisingly. That is, the heat transfer can be raised only by adding an appropriate amount of additives inside the heated container with vibration or reciprocating rotation, then the boiling heat transfer can be raised by 20∼65% without increasing the heating area.     

Pressure Drop,Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m²s and heat fluxes from 200 to 500 kW/m². The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.     

Design and Testing of a Graphite Foam-Based Supercooler for High-Heat-Flux Cooling in Optoelectronic Packages

The feasibility of using graphite foam as a heat sink and heat spreader in optoelectronic packages is assessed. A “supercooler” is designed, fabricated, and tested to verify its cooling capability under high heat flux conditions in a typical optoelectronic package. The supercooler uses graphite foam as a primary heat transfer material. Water is soaked into the graphite foam, and under evacuated pressure, boiling is initiated under the heating region to provide enhanced cooling. Experiments were conducted for a heat flux of up to 400 W/cm² deposited over a heating area of 0.5 mm × 5 mm. Two-dimensional transient temperature distributions were recorded using a high-speed infrared camera. Data were obtained for steady heating, and for periodic heating with frequency up to 8 Hz. Results show that the supercooler is very efficient in dissipating heat away from the heating region. The average cooling rate during the cooling period exceeds 170 K/s.     

Enhanced Boiling Heat Transfer by Using Micro-Pin-Finned Surface in Three Different Test Systems

For efficiently cooling electronic components, experiments were conducted to study heat transfer performance of FC-72 over a silicon chip with micro-pin-fins in three different test systems including pool boiling, natural circulation flow boiling, and forced flow boiling. A smooth chip (chip S) and a micro-pin-finned chip having fin thickness of 30 μm and fin height of 60 μm (chip PF30-60) were tested. The micro-pin-fins were fabricated on the surface of square silicon chip (10 × 10 × 0.5 mm³) by use of dry etching technique for enhancing boiling heat transfer. The experiments were conducted at fluid velocities of 0.5 and 1 m/s for the forced flow boiling and the liquid subcoolings of 25 and 35 K for all three test systems. With regard to the three test systems, the micro-pin-finned surface shows a considerable heat transfer enhancement compared to the smooth surface and shows a sharp increase in the critical heat flux (CHF). For the same chip, the boiling curves are almost the same for the pool boiling and the natural circulation flow boiling, while the boiling curves shift toward a smaller wall superheat for the forced flow boiling. The critical heat flux was the highest for the forced flow boiling at a fluid velocity of 1 m/s.     

Static heat transfer to liquid helium in open pools and narrow channels

The transfer of heat to boiling liquid helium has been measured in open pools and narrow channels. In open pools a marked dependence of heat transfer on the orientation of the heated surface is observed. The maximum heat flux for nucleate boiling varies from 1 W/cm² with the heated surface horizontal facing upwards to about 0·1 W/cm² with the surface horizontal facing downwards. In a narrow vertical channel the maximum heat flux is reduced to about 0·15 W/cm² for a rectangular channel 10mm × 1 mm (50 cm length), and appears to decrease linearly with the channel dimension. The heat transfer is considerably increased in the narrow channel when the fluid is pressurized.     

Performance of LHPs with a novel design evaporator

In order to realize an excellent heat transfer performance of the LHPs, including the fast start-up and high heat transfer capacity, a new connection design between the evaporator envelope and the wick surface without the clearance was proposed. The LHPs with a cylindrical evaporator, 22 mm diameter and 80 mm long, were fabricated with water as the working fluid and an 70% inventory.Copper wicks made of different particle sizes were used in both the start-up and heat transfer capacity tests. It was experimentally observed that the sintered wick with 139 μm diameter particles had the best heat transfer performance. It achieved a start-up time of only 150 s under 30 W heat load, a heat transfer capacity of 500 W under the allowable evaporator temperature of 85 °C, and a low thermal resistance of 0.070–0.165 °C/W. Internal temperature measurements were also conducted to determine the mechanism of the heat leak, to identify the heat pipe effect, and to compare the heat leak with different wicks corresponding to the change of the heat load during the operation