Protruding Heat Sources Research Articles

Natural convection to air from an array of vertical parallel plates with discrete and protruding heat sources

Numerical and experimental studies are carried out on natural convection heat transfer to air from an array of vertical parallel plates with protruding and discrete heat sources, such as IC packages. Two-dimensional (2-D) Navier-Stokes and energy equations including heat conduction through the plates and heat sources are numerically solved under the conditions of modified Grashof number Gr ∗ = 2.3 × 103 ∼ 8.8 × 10 5 and plate height to spacing ratio L = l h = 8 ∼ 30 . The numerical solutions agree well with the experimental values except for the large aspect ratio L = 30, where the three-dimensional (3-D) effects become significant. A correlation expression for the local Nusselt number is proposed; it can predict the protrusion surface temperature within ±20% error. A method for estimating the maximum inner temperature of the heat source is proposed, and the thermally optimum spacing of the parallel plates is discussed.

Conjugate Heat Transfer in a Vertical Channel with Protrunding Heat Source

The coupled conduction and convection heat transfer from the protruding heat source in a vertical channel is numerically investigated. Conjugate solution of the two-dimensional energy equation is obtained for the incompressible air flow over the rectangular block with local heat source. It was found that several recirculation zones and separation bubble near the block were related to Re and Gr. And the results show that fractions of the heat transfer through each of the block face, maximum temperature of the block and the relative effect of each parameter on the maximum temperature and heat transfer.

An experimental and numerical study on natural convection-radiation conjugate heat transfer in a three-dimensional enclosure having a protruding heat source

An experimental and numerical study on the three-dimensional natural convection-radiation conjugate heat transfer in the enclosure with heat generating chip has been performed. A 3-dimensional simulation model is developed by considering heat transfer phenomena by conduction-convection and radiation. Radiative transfer was analyzed with the discrete ordinates method. Experiments are conducted in order to validate the numerical model. Comparisons with the experimental data show that good agreement is obtained when the radiation effect is considered. The effects of the thermal conductivity of the substrate and power level on heat transfer are investigated. It is shown that radiation is the dominant heat transfer mode and the conductivity of the substrate has important effects on the heat transfer in the enclosure.

CONJUGATE NATURAL CONVECTION FROM AN ARRAY OF PROTRUDING HEAT SOURCES

Abstract Coupled conduction and natural convection heat transfer has been investigated numerically for protruding heat sources mounted to one vertical wall of a rectangular cavity. The heat sources simulate an array of computer chips mounted on a substrate of finite thermal conductivity. The back of the substrate and the horizontal walls of the cavity are assumed to be adiabatic, while the opposing vertical wall provides an isothermal heal sink. The fluid Prandtl number and the heater/fluid thermal conductivity ratio are fixed at 25 and 2350, respectively, corresponding to a dielectric fluid (FC-77, manufactured by 3M Company) and silicon chips. With increasing modified Rayleigh number (104 < Ra∗ LZ < 109), the cavity flow approaches boundary layer behavior, and more fluid penetrates the regions between protrusions. The effect of contact resistance between the heater and substrate is shown to be small for Rn th < 10 cm2 °C/W. With decreasing substrate thermal conductivity [1.48 W/ (m K) <. Ks < 148 W/(m K...

Natural Convection to Air from an Array of Vertical Parallel Plates with Discrete and Protruding Heat Sources.

Numerical and experimental studies are carried out on natural convection heat transfer to air from an array of vertical parallel plates with protruding and discrete heat sources such as IC packages. Two-dimensional Navier-Stokes and energy equations including heat conduction through the plates and heat sources are numerically solved under the conditions of modified Grashof number Gr=2.3×103∼8.8×105 and aspect ratio L=l/h=8∼30. The numerical solution agree well with the experimental values except for the large aspect ratio L=30 where the three-dimensional effects become significant. A correlation expression for the local Nusselt number is proposed which can predict the protrusion surface temperature within 20% error. The estimation of the protrusion center temperature is also discussed.

OPTIMAL SPACING OF PARALLEL BOARDS WITH DISCRETE HEAT SOURCES COOLED BY LAMINAR FORCED CONVECTION

This paper shows numerically how to select the optimal spacing between boards mounted in a stack of specified volume, so that the overall thermal conductance between the stack and the forced coolant is maximum. Several configurations are considered: boards with uniform flux, flush-mounted discrete sources, and protruding heat sources. The flow is laminar and the pressure difference across the stack is fixed (Δp). It is shown that for all the board geometries and thermal boundary conditions studied, the optimal board-to-board spacing is correlated by (Dopt/l ∼ 2.7(Δpl2 / μα)−¼where l is the effective longitudinal (flow) distance occupied by the discrete sources and the unhealed patches contained between them, and μ and α are the viscosity and thermal diffusivity of the fluid. If U∞is the free-stream velocity upstream of the stack, the optimal spacing is given by (Dopt/l) ∼ 3.2pr−¼(U∞l / v)−½.

Computations of liquid immersion cooling for a protruding heat source in a cubical enclosure

Three-dimensional natural convection flow and heat transfer were studied numerically for a heated, protruding substrate-mounted chip or package in a cubic enclosure filled with a dielectric liquid. The governing steady-state equations for the natural convection within the liquid and the conjugate conduction within the element and the substrate were solved using a finite-volume method over a wide range of Rayleigh numbers ( Ra) and substrate-to-fluid thermal conductivity ratios ( R s). Conduction through the substrate plays a dominant role in the cooling of the chip for values of R s, greater than 10. Comparison of the numerical results with experimental data showed reasonable agreement, particularly in the patterns of isotherms observed on the substrate surface.

Heat Transfer and Flow Characteristics of Two-Dimensional Jets Impinging on Heated Protrusions With Crossflow of the Spent Air

The flow structure and local heat transfer characteristics of two-dimensional slot jets impinging on heated protrusions has been investigated. The spent air was constrained to exit at one end of the channel, forming a crossflow. The effects of three parameters on the heat transfer were examined for an array of five protruding heat sources. They include the jet slot width, distance between the jet exit and the protrusion, and the average jet Reynolds number. Laser-Doppler velocimeter measurements were made to detail the mean and turbulent flow structure in the channel. Experimental results reveal that the flow and heat transfer are dominated by turbulent transport even for Reynolds numbers as low as 300. Two transport mechanisms were identified affecting the heat transfer. The first was jet impingement, the second being crossflow of the spent air. A complex interaction between the two mechanisms was observed. At low nozzle-protrusion spacing with large slot jets the heat transfer was dominated by the crossflow, whereas for high nozzle-protrusion spacing and small jets, transport was dominated by jet impingement. It is postulated that the highest average Nusselt number occurs when the jets and the crossflow influence act with near-equal intensity.

The Aspect Ratio Effect on Natural Convection in an Enclosure With Protruding Heat Sources

The aspect ratio effect on natural convection heat transfer in a rectangular enclosure with protruding heat sources has been experimentally investigated. Five protruding heaters were mounted with uniform vertical spacing on one vertical wall. The vertical wall opposite to the wall on which heated sections were mounted was movable so that the enclosure width could be adjusted to the desired value. The top surface of the test enclosure was an isothermal heat sink. All other surfaces except the two end vertical surfaces were insulated. The five heaters were identical with each having horizontal protuberance of L3 = 9 mm and vertical height of L1 = 15 mm. The vertical spacing between the heaters was L2 = 15 mm. The enclosure width was varied in experiments from W = 13.5 mm to 45 mm. The experiments were conducted for six values of cavity width resulting in variations in the cavity height-to-width ratios (aspect ratios) and cavity width-to-protruding heater height ratios of 3.67 to 12.22 and 1.5 to 5.0, respectively. Ethylene glycol was used as the convective medium. Flow visualization pictures and heat transfer data indicate that the starting point of core flow directly affects the local heat transfer coefficient of the bottom heater, while the secondary flow cell between the top heated section and the top sink surface influences the heat transfer coefficient of the top heater. Cavity width variation influences the heat transfer process mainly through altered flow patterns. This influence is weak when the ratio of width-to-protruding height is 4.0 and negligible when this ratio is 5.0 or more. Based on the local height length scale (measured from the bottom of the cavity) the data for all the cavity widths are correlated and an explicit relation for the aspect ratio effect on local Nusselt number is reported. The correlation of local Nusselt number versus local modified Rayleigh number is independent of the number of heaters in the vertical array, cavity width-to-heater protrusion height ratio, W/L3, and vertical height location of the heaters. This conclusion is based on the present results and previously reported data and is valid for the following conditions: 1.5 ≤ W/L3 ≤ 5.0; 3.67 ≤ aspect ratio ≤ 1.22; vertical height of heater from 8 to 15 mm; and number of heated sections from 5 to 10.

Natural convection arising from a heat generating substrate-mounted protrusion in a liquid-filled two-dimensional enclosure

An investigation of natural convection flow and heat transfer arising from a substrate-mounted protruding heat source immersed in a liquid-filled square enclosure is reported. The model considers heat transfer within the protrusion and substrate and the coupled natural convection in the fluid. Numerical predictions are obtained for a wide range of appropriate Rayleigh and Prandtl numbers and substrate to fluid thermal conductivity ratios that may be encountered in liquid-immersion cooling of electronic components. For many situations of interest prescribing simplistic heat transfer conditions at the solid surfaces is found inappropriate. Increasing the Rayleigh number beyond 106 and the substrate thermal conductivity beyond 100 times that of the liquid produces only a marginal decrease in the maximum temperatures. Computed protrusion surface temperatures compare favorably with available experimental results for a similar configuration.

Local forced convective heat transfer from protruding and flush-mounted two-dimensional discrete heat sources

Experiments are undertaken to measure the local forced convective heat transfer characteristics of an array of two-dimensional discrete heat sources. Air flow rates yielding effective channel Reynolds numbers in the range from 1000 to 10000 are employed. Flush-mounted and protruding heater configurations are investigated in the study. Two different channel wall spacings independent of the heat source protrusion are also studied. Local temperature measurements are made along the uniformly heated surface parallel to the flow direction. The results of the experiments show that protruding heat sources yield higher heat transfer than flush-mounted heat sources at the same channel Reynolds number. The interruption of the thermal boundary layer in the adiabatic sections between the heaters results in heat transfer enhancement. Heat source protrusion yields significantly higher heat transfer rates for the second and subsequent heater for channal Reynolds numbers above 2000. This is explained in terms of a separated flow and vena contracta effect, with eventual downstream transition to turbulent flow, and is borne out by flow visualization experiments. The local heat transfer measurements reveal significant variation in wall temperature across the heater faces, which is strongly affected by the flow structure. Average heat transfer coefficients calculated by spatially integrating the local measurements are correlated and presented in terms of Reynolds number and the dimensionless geometric parameters studied.

Mixed convection transport from a protruding heat source module on avertical surface

An experimental study of mixed convective transport from a protruding heat source module mounted on a vertical surface in an externally induced flow has been carried out. This problem is of particular interest in the design of thermal systems, such as the cooling of electronic circuitry and positioning of a heating element in a furnace. The temperature and velocity distributions are measured. The dependence of the heat transfer rate and of the thermal field on the mixed convection parameter and on the thickness of the heat source module, particularly in the vicinity of the source, are investigated in detail. Experimental results indicate that the flowfield and the temperature distribution in the flow over the module are substantially affected by the module thickness and also by the mixed convection parameter due to the combined effects of the forced convection flow and of the buoyancy-driven flow arising from the heat input. These, in turn, have a strong influence on the convective heat transfer from the heat source to the ambient. The thermal and viscous boundary layers as well as the variation of the surface temperature are determined and the results discussed in terms of underlying physical processes. The results obtained are also compared with those for an element of negligible thickness. Thus, the effect of a significant module thickness on the convective transport is determined.

Natural convection heat transfer characteristics of a protruding thermal source located on horizontal and vertical surfaces

An experimental study of natural convection heat transfer from a heat source module of finite thickness, mounted on a vertical or horizontal surface, is carried out. This problem is of particular interest in the removal of thermal energy from heated elements in manufacturing and electronic systems. A wall plume, adjacent to the surface, arises if the heat source module is mounted on a vertical surface. A freely rising thermal is generated by the heat source when it is located on a horizontal surface. The temperature distributions in the flow and along the surface are measured. The results obtained indicate that the natural convection flow and the associated heat transfer characteristics vary strongly with the rate of energy input and the source thickness. A comparison between the free and wall plume circumstances indicates very different temperature distributions. While the resulting isotherms for the free plume case are symmetric about the source, those for the wall plume case are swept downstream of the source. It is found that conduction along the plate length is of considerable importance in the transport process. The downstream locations of the plate come under the effect of the wake for the wall plume case, whereas the region far from the source is free from the wake effect in the free plume circumstance which arises in the horizontal orientation. The interaction between the wakes generated by the three exposed surfaces of the heat source module is also found to have a significant effect on the flow. Thus, the heat transfer from a protruding heat source module cannot be accurately determined by assuming three heated, isolated surfaces. The results obtained are compared with those for a source of negligible thickness and the effect of a significant module thickness on the convective flow and transport is determined. The heat transfer coefficient for a protruding heat source module is found to depend on the ratio of the vertical to the horizontal surface areas of the module, exposed to the ambient medium. Several other interesting trends are observed in this investigation and discussed in terms of the underlying flow and thermal transport processes.

Laminar mixed convection in a horizontal channel with protruded heat sources

Abstract The problem of laminar mixed convection in a horizontal dimensional channel with one or two protruded heat sources on one of the walls is studied numerically. The top surface of the protruding heat source is assumed to be isothermal while the rest of the surfaces and the walls are assumed to be adiabatic. The governing dimensionless equations in terms of vorticity, stream function and temperature are solved numerically by the cubic spline collocation method. The effects of the mixed convection parameter, Gr/Re 2, height of thermal sources, D/H, and the interaction of the protruded heat sources on fluid and heat transfer characteristics of air in a horizontal channel are presented. The results show that the heat transfer rates increase as the Gr/Re 2 and D/H are increased because of the increase of the buoyancy effects. The results also show that the effects of the interaction of the two protruding heat sources are remarkably affected by the flow field and temperature field in the horizontal channel.