竖直矩形窄通道内饱和沸腾起始点的研究

以去离子水为工质,对尺寸为720 mm×250 mm×3.5 mm的单面电加热竖直矩形窄通道内饱和沸腾起始点进行实验研究。分析了加热热流密度、工质进口温度和质量流量对饱和沸腾起始点位置及饱和沸腾起始点处壁面过热度的影响。在已有饱和沸腾起始点预测关联式的基础上,对实验数据进行非线性回归分析,得到适用于单面加热矩形窄通道饱和沸腾起始点的新关联式。结果表明：新拟合的关联式预测值与实验值的平均相对误差为17.63%,能很好的预测常压、低加热热流密度与低流速条件下的饱和沸腾起始点处壁面过热度与热流密度的关系。     

Two-Phase Heat Sinks with Microporous Coating

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy based and was sprayed on heat sink channels, while the second coating was formed by sintering copper particles on heat sink channels. Minichannel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and 12 rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for minichannel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from ～6°C to slightly more than 1°C. No increase in pressure drop or pumping power due to the microporous coating was measured. The minichannel heat sinks with porous coating had on average 1.5 times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had on average 2.5 times higher heat transfer coefficient and the critical heat flux was 1.5 to 2 times higher compared with the uncoated heat sinks.     

Flow Boiling in an Inclined Channel With Downward-Facing Heated Upper Wall

Wall boiling and bubble population balance equations combined with a two-fluid model are employed to predict boiling two-phase flow in an inclined channel with a downward-facing heated upper wall. In order to observe the boiling behavior on the inclined, downward-facing heated wall, a visualization experiment was carried out with a 100 mm × 100 mm of the cross section, 1.2-m-long rectangular channel, inclined by 10° from the horizontal plane. The size of the heated wall was 50 mm by 750 mm and the heat flux was provided by Joule heating using DC electrical current. The temperatures of the heater surface were measured and used in calculating heat transfer coefficients. The wall superheat for 100 kW/m² heat flux and 200 kg/m²s mass flux ranged between 9.3°C and 15.1°C. High-speed video images showed that bubbles were sliding, continuing to grow, and combining with small bubbles growing at their nucleation sites in the downstream. Then large bubbles coalesced together when the bubbles grew too large to have a space between them. Finally, an elongated slug bubble formed and it continued to slide along the heated wall. For these circumstances of wall boiling and two-phase flow in the inclined channel, the existing wall boiling model encompassing bubble growth and sliding was improved by considering the influence of large bubbles near the heated wall and liquid film evaporation under the large slug bubbles. With this improved model, the predicted wall superheat agreed well with the experimental data, while the RPI model largely overpredicted the wall superheat.     

Study on heat transfer characteristics of porous metallic heat sink with conductive pipe under bypass effect

The work investigated the forced convection heat transfer of the heat sink situated in a rectangular channel by considering the bypass effect. The fluid medium was air. The relevant parameters were the Reynolds number (Re), the relative top by‐pass gap (C/H), and the relative side by‐pass gap (S/L). The size of the heat sink was 60 mm (L)×60 mm(W)×24 mm(H). Two heat sinks were employed as test specimens: (A) the 0.9‐porosity aluminum foam heat sink and (B) the 0.9‐porosity aluminum foam heat sink with a 20 mm diameter copper cylinder. The copper cylinder was used as a conductive pipe of heat sink. The average Nusselt number was examined under various forced convection conditions. Experimental results demonstrate that increasing by‐pass space decreased the Nusselt number. Besides, the average Nusselt number of mode B heat sink was higher than that of mode A heat sink by 30% for the case without by‐pass flow. The heat transfer enhancement by the copper cylinder would decline as the by‐pass space grew. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20247     

Critical heat flux in non‐uniformly heated tube under low‐pressure and low‐mass‐flux condition

In an actual boiling channel, e.g., a boiler water‐tube, the circumferential heat flux is not uniform. Thus, the critical heat flux (CHF) of a non‐uniformly heated tube becomes an important design factor for conventional boilers, especially for a compact water‐tube boiler with a tube‐nested combustor. A small compact boiler is operated under low‐pressure and low‐mass‐flux conditions compared with a large‐scale boiler, thus the redistribution of liquid film strongly affects the characteristics of CHF. In this investigation, non‐uniform heat flux distribution along the circumferential direction was generated by using the Joule heating of SUS304 tubes with the wall thickness distribution. The heated length of test‐section was 900 mm with an inner diameter of 20 mm and an outer diameter of 24 mm. The center of the inner tube surface was shifted by ε=0, 0.5, 1.0, 1.5 mm from the center of the outer tube surface. The heat flux ratio between maximum and minimum heat flux of these tubes corresponded to 1.0, 1.7, 3.0, and 7.0, respectively. The experimental conditions were as follows: system pressure at 0.3 and 0.4 MPa, mass flux of 10–100kg/(m²s), inlet temperatures at 30° and 80°. The experimental results showed an increase in the critical heat flux substantiated by the existence of the redistribution of the flow. These characteristics are explained by using a concept similar to that of Butterworth's spreading model. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 47–60, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20095     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Enhancement of natural convection heat transfer from a vertical heated plate using inclined fins

An enhancement technique is developed for natural convection heat transfer from a vertical heated plate with inclined fins, attached on the vertical heated plate to isolate a hot air flow from a cold air flow. Experiments are performed in air for inclination angles of the inclined fins in the range of 30° to 90° as measured from a horizontal plane, with a height of 25 to 50 mm, and a fin pitch of 20 to 60 mm. The convective heat transfer rate for the vertical heated plate with inclined fins at an inclination angle of 60° is found to be 19% higher than that for a vertical heated plate with vertical fins. A dimensionless equation on the natural convection heat transfer of a vertical heated plate with inclined fins is presented. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 334–344, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20168     

CHF in a circumferentially non‐uniformly heated tube under low‐pressure and low‐mass‐flux condition (inclined upward flow)

Most investigations on forced convective boiling have been conducted by using uniformly heated round tubes under a vertical upward flow condition, although the actual system has a non‐uniformly heated condition with several tube orientations. The non‐uniformity of the heat flux and tube inclination causes the liquid film distribution, which in turn affects the critical heat flux. In this investigation, the flow and heat‐transfer characteristics were experimentally investigated under non‐uniformly heated conditions along the circumferential direction with a 45° tube inclination. In the experiment, CHF was measured by using two different heated lengths, i.e., 900 and 1800 mm. The experimental results showed a unique tendency of CHF caused by the interrelationship of the non‐uniform heat flux distribution, the tube inclination, and liquid film redistribution. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20333     

Combined Effect of Mixed Convection and Surface Radiation Heat Transfer for Thermally Developing Flow in Vertical Channels

Combined effect of laminar flow mixed convection and surface radiation heat transfer for thermally developing airflow in a vertical channel heated from a side has been experimentally examined with different thermal and geometric parameters. The channel boundary is made of two isothermal walls and two adiabatic walls, the isothermal parallel wall is heated uniformly and the opposite cold wall temperature is maintained equal to the inlet conditions. The heated wall temperature ranged from 55 to 100°C, Reynolds number ranged from 800 to 2900 and the heat flux was varied from 250 to 870 W/m². To cover the wide range of Reynolds numbers, two aspect ratios of square and rectangular section were used. Surface radiation from the internal walls is considered through two emissivities i.e. 0.05 and 0.85, to represent weak and strong radiation effects, respectively. From the experiments, surface temperature and Nusselt number distributions of convection and radiation heat transfer are obtained for different heat flux values. Flow structure inside the channel is visualized to observe the flow pattern. The results show the combined effect of laminar flow mixed convection and surface radiation on the total heat transfer rate within the channel. The accumulating buoyancy force and airflow moves together vertically in the upward direction to give significant heat transfer enhancement in the vertical orientation of the channel.     

Nucleate Boiling Enhancements on Porous Graphite and Microporous and Macro–Finned Copper Surfaces

Enhancements in nucleate boiling heat removal with dielectric liquids, by increasing either the bubbles nucleation sites density and/or the wetted surface area, are desirable for immersion cooling of high-power computer chips. This article presents the results of recent investigations of nucleate boiling enhancement of FC-72, HFE-7100, and PF-5060 dielectric liquids on porous graphite, copper microporous surfaces, and copper surfaces with square corner pins, 3 mm × 3 mm in cross-section and 2, 3, and 5 mm tall. All surfaces have a footprint measuring 10 × 10 mm. These investigations examined the effects of liquid subcooling up to 30 K and surface inclination, from upward-facing to downward-facing, on nucleate boiling heat transfer coefficient and critical heat flux. Natural convection of dielectric liquids for cooling chips while in the stand-by mode, at a surface average heat flux <20 kW/m², is also investigated for the different surfaces.     

Fluid flow and heat transfer of natural convection around large horizontal cylinders: Experiments with air

Natural convective flows of air around large horizontal cylinders were investigated experimentally. The main concerns were the turbulent transition mechanisms and the heat transfer characteristics of turbulent flows. The cylinders were heated with uniform heat flux and their diameters were varied from 200 to 1200 mm to enable experiments over a wide range of modified Rayleigh numbers, Ra_D^* = 1.0 × 10⁸ to 5.5 × 10¹¹. The flow fields around the cylinders were visualized with smoke to investigate the turbulent transition mechanisms. The results show that three‐dimensional flow separations occur first at the trailing edge of the cylinder when Ra_D^* exceeds 3.5 × 10⁹, and the separation points shift upstream with increasing Rayleigh numbers. These separations become a trigger to the turbulent transition and transitional and turbulent flows appear downstream of the separations at higher Rayleigh numbers. However, they occupy a relatively small portion of the cylinder surfaces even at the maximum Rayleigh numbers of the present experiments. The local heat transfer coefficients were also measured. The results show that the coefficients are increased significantly in the transitional and turbulent regions compared with the laminar coefficients. Moreover, the present results for air were compared with previous results for water and the effects of Prandtl number on the flow and heat transfer were discussed. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 293–305, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10080     

Subcooled Flow Boiling in a Minichannel

It has been considered that dry-out occurs easily in boiling heat transfer for a small channel, a mini- or microchannel, because the channel was easily filled with coalescing vapor bubbles. In the present study, the experiments of subcooled flow boiling of water were performed under atmospheric conditions for a horizontal rectangular channel for which the size is 1 mm height and 1 mm width, with a flat heating surface of 10 mm length and 1 mm width placed on the bottom of the channel. The heating surface has a top of copper heating block and is heated by ceramic heaters. In the high heat flux region of nucleate boiling, about 70–80% of the heating surface was covered with a large coalescing bubble and the boiling reached critical heat flux as observed by high-speed video. In the beginning of transition boiling, coalescing bubbles were collapsed to many fine bubbles and microbubble emission boiling was observed at liquid subcooling higher than 30 K. The maximum heat flux obtained was 8 MW/m² (800 W/cm²) at liquid subcooling of higher than 40 K and a liquid velocity of 0.5 m/s. However, the surface temperature was very much higher than that of a centimeter-scale channel. The high-speed video photographs indicated that microbubble emission boiling occurs in the deep transition boiling region.     

Visualization and thermal resistance measurement for the sintered mesh-wick evaporator in operating flat-plate heat pipes

This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26 mm to 0.8 mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100 W/cm². Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances.     

Free convection of composite porous medium in a vertical channel

A fully developed free convection flow of immiscible fluids in a vertical channel filled with a porous medium is analyzed in the presence of source/sink. The flow is modeled using the Darcy–Brinkman–Forchheimer equation model. The viscous and Darcy dissipation terms are included in the energy equation. The channel walls are maintained at two different constant temperatures. The transport properties of both fluids are assumed to be constant. Continuous conditions for velocity, temperature, shear stress, and heat flux of both fluids at the interface are employed. The resulting coupled nonlinear equations are solved analytically using regular perturbation method and numerically using finite difference method. The velocity and temperature profiles are obtained in terms of porous parameter, Grashof number, viscosity ratio, width ratio, conductivity ratio, and heat generation or heat absorption coefficient. It is found that the presence of porous matrix and heat absorption reduces the flow field. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20340     

Numerical Study of Transient Natural Convection in Air in Vertical Divergent Channels

This article carries out a numerical, transient, two-dimensional analysis of natural convection in air in a divergent channel, characterized by two inclined flat plates heated at a uniform heat flux. The flow is assumed to be laminar and incompressible. Simulations allow to detect the complex structures of the flow inside and outside the channel. Results, in terms of temperature distributions, average Nusselt and Reynolds profiles, depending on time as a function of the divergence angle and channel spacing, are presented. Flow visualizations and stream function contours confirmed the disturbances inside the channel for the highest divergence angles (>5°). Correlations in terms of Nusselt numbers as a function of Rayleigh and Rayleigh modified numbers, ranging from 7.6 × 10² to 1.3 × 10⁹ and from 30 to 8.2 × 10⁸, respectively, were proposed. They were in very good agreement with the experimental relations.     

The effects of geometrical parameters of a corrugated channel with in out-of-phase arrangement

Numerical investigations of forced turbulent convective flow and heat transfer in a corrugated channel of plate heat exchanger are carried out. The continuity, momentum and energy equations were solved by means of a finite volume method (FVM). The top and bottom walls of the corrugated channel are heated at constant heat flux boundary conditions. The effects of geometrical parameters of corrugated tilt angles, channel heights and wavy heights using water as a working fluid on the thermal and flow fields as well as on the performance of evaluation criterion are studied. The corrugated channel with three different corrugated tilt angles of 20°, 40° and 60° with different channel heights of 12.5, 15 and 17.5 mm and different wavy heights of 2.5, 3.5 and 4.5 mm are tested. This investigation covers Reynolds number and heat flux in the range of 8000–20,000 and 0.4–6 kW/m², respectively. The numerical results indicate that the wavy angle of 60° and wavy height of 2.5 mm with channel height of 17.5 mm are the optimum parameters and they have a significant effect on the heat transfer enhancement. It is found that using wavy channel is a suitable method to increase the thermal performance and getting higher compactness of the heat exchanger.     

Confined Boiling Heat Transfer,Two-Phase Flow Patterns,and Jet Impingement in a Hele-Shaw Cell

ABSTRACT

Experiments were carried out to study heat transfer and two-phase flow patterns during boiling in a Hele-Shaw cell filled with pure water vapor at atmospheric pressure and with a central inlet of a liquid jet. The Hele-Shaw cell was based on a circular copper rod surface and a polycarbonate plate permitting optical access and thus high-speed cinematography. The diameter of the heated copper rod was 10 mm, the jet diameters were 0.5 and 1 mm, and spacing was varied between 50, 100, and 200 μm. The heat was applied through 4 cartridge heaters with a maximum heat flux of 327 W/cm². Results showed how high-volume flow rates for the liquid jet led to jet impingement heat transfer while low flow rates led to a Hele-Shaw flow boiling system. The relationship between the volume flow rate and the temperature difference differed significantly between these two regimes. Different flow patterns and evaporation fronts were observed using high-speed cinematography. They strongly depended on jet properties, applied heat flux, and gap spacing. The efficiency of the Hele-Shaw flow boiling system during high heat flux levels was attributed to high interface velocities, combined with viscous fingering at the interface. This combination led to high wetting rates with substantial microlayer evaporation. Good results regarding the heat transfer and the pressure drop were obtained with the final configuration of a 10-mm copper rod diameter, a jet diameter of 1 mm, and a spacing of 0.1 mm. A rather surprising observation was the existence of a stable rotation of an evaporating liquid jet in the Hele-Shaw boiling chamber. The driving mechanism for the rotation with a frequency of 105 Hz was the rapid microlayer evaporation at the rear side of the rotating liquid jet.     

Effect of porous coating on critical heat flux

The critical hear flux (CHF) is studied experimentally in vertical tubes heated directly using power current (DC 2500 A, 15 V) and cooled with water at a low mass flow rate (0 ∼ 200 kg/m²s) and at a low pressure (1.0 ∼ 8.0 bar). We used a smooth tube and a tube with a porous coating layer sintered onto the inner surface. The results are compared with each other and with correlations by Katto [1] and Weber [2]. We determined enhancement of heat transfer as well as a negative effect of the porous coating below the expected value of critical heat flux.     

Forced Convective Heat Transfer in a Porous Plate Channel

INTRODUCTIONHeattransferenllancen1enttechniquesplayaveryimportantroleintllermalcontroltechnologies1lsedwithnlicroelectronicchips,powerfullasermirrors,aerospacecraft,thermalnuclearfusion,etc.Itiswidelyrecognizedthattl1eheattransfercanbein-creasedbyil1creasingthesurfaceareaincontactwiththecoolant.TuckermanandPease[1,2]pointedoutthatforlaminarflowinconfinedchannels,theheattransfercoefficientisinverselyproportionaltothewidthofthechannelsincethelimitingNusseltnum-berisconsta11t.Theybuiltawate…     

Visualization and Modeling of Flow Boiling of R-134A in Minichannels with Transverse Ribs

This research was undertaken to investigate the flow boiling of R-134a in minichannels with transverse ribs. The test channel consisted of two parallel plates; the lower plate was heated, and the upper plate was adiabatic. A number of transverse ribs were deployed along the lower plate. The hydraulic diameter of the channel was 1.93 mm. In these experiments, the main test parameters were mass flux, saturation temperature, heat flux, and average vapor quality. With these parameters, the convective and boiling numbers ranged from 0.04 to 0.90 and 7.49 × 10^{? 4} to 8.98 × 10^{? 4} , respectively. Of a total of 60 data points recorded, 78% of the data fell in the convective boiling region, while the remaining 22% were in the nucleate boiling region. A two-equation model was fit to a total of 60 experimental data, resulting in a correlation that predicted 58 data points to within ± 10%.