John E. Leland

Experimental Investigation of Oscillating Heat Pipes

Operation requirements of oscillating heat pipes (OHPs) are proposed. Based on the requirements, OHPs with nonflammable fluorocarbon fluids, FC-72 and FC-75, as the working fluid are developed. The OHPs have an inner diameter of 1.75 mm, a total length of 446 mm, and 40 tubing turns. There are two condensers on both outer sides and one evaporator in the middle of the OHPs. Thermal performance tests are conducted at various operating temperatures and heat rates. The working fluid fill ratio is varied. A high-performance OHP with FC-72 has been indicated for the first time. The FC-72 OHP can transport a 2040-W heat rate without dryout. The gravitational acceleration does not have a noticeable influence on the performance of the fluorocarbon OHP. The thermal performances of the fluorocarbon OHPs are compared with the case of an acetone OHP.

NUMERICAL MODELING OF CONJUGATE HEAT TRANSFER DURING IMPINGEMENT OF FREE LIQUID JET ISSUING FROM A SLOT NOZZLE

The conjugate heat transfer from discrete heat sources to a two-dimensional jet of a high Prandtl number fluid discharging from a slot nozzle is considered. The variation of solid and fluid properties with temperature was taken into account in the numerical simulation. The geometry of the free surface was determined iteratively. The influence of different operating parameters such as jet velocity, heat flux, plate thickness, plate material, and the location of the heat generating electronics were investigated. It was found that in addition to jet Reynolds number (Re) plate thickness and its thermal conductivity have significant influence on temperature distribution and average Nusselt number (Nu).The conjugate heat transfer from discrete heat sources to a two-dimensional jet of a high Prandtl number fluid discharging from a slot nozzle is considered. The variation of solid and fluid properties with temperature was taken into account in the numerical simulation. The geometry of the free surface was determined iteratively. The influence of different operating parameters such as jet velocity, heat flux, plate thickness, plate material, and the location of the heat generating electronics were investigated. It was found that in addition to jet Reynolds number (Re) plate thickness and its thermal conductivity have significant influence on temperature distribution and average Nusselt number (Nu).

Analysis of Transient Conjugate Heat Transfer to a Free Impinging Jet

Transient conjugate heat transfer during the impingement of a freejet of high Prandtl number e uid on a solid disk of e nite thickness is considered. When power is turned on at t =0, a uniform heat e ux is imposed on the disk surface. The numerical model considers both solid and e uid regions. Equations for the conservation of mass, momentum,andenergy aresolvedintheliquidregion,withthetransportprocessesattheinletand exitboundaries, as well as at the solid ‐liquid and liquid ‐gas interfaces taken into account. In the solid region, only heat conduction equation is solved. The shape and location of the free surface (liquid‐gas interface ) is determined iteratively as a part of the solution process by satisfying the kinematic condition as well as the balance of normal and shear forces at this interface. Computed results include the velocity, temperature, and pressure distributions in the e uid and the local and average heat transfer coefe cients at the solid ‐e uid interface. Computations are carried out to investigate the ine uence of different operating parameters such as jet velocity, disk thickness, and disk material. Nomenclature b = thickness of the disk, m cp = specie c heat at constant pressure, kJ/kg ¢ K dn = diameter of the nozzle, m Fo = Fourier number, a ft/d 2 n g = acceleration due to gravity, m/s 2 Hn = distance of the nozzle from the disk, m h = heat transfer coefe cient, qint/(Tint i Tj), W/m 2 ¢ K hav = heat transfer coefe cient, W/m 2 ¢ K:

Conjugate Heat Transfer During Free Jet Impingement of a High Prandtl Number Fluid

This article presents the results of numerical simulation of a free jet of a high Prandtl number fluid impinging perpendicularly on a solid substrate of finite thickness containing electronics on the opposite surface. The numerical model was developed considering both solid and fluid regions and solved as a conjugate problem. The influence of different operating parameters such as jet velocity, heat flux, plate thickness, nozzle height, and plate material were investigated. Computed results were validated with available experimental data. It was found that the local Nusselt number is maximum at the center of the disk and decreases gradually with radius as the flow moves downstream. The average Nusselt number and the maximum temperature occurring in the solid varied significantly with impingement velocity, disk thickness, and thermal conductivity of the disk material.

Experimental Investigation of an Air Microjet Array Impingement Cooling Device

A microjet impingement cooling device for high power electronics was constructed from silicon wafers using microelectromechnical systems fabrication techniques. The array of 221, 0.277-mm-diam jets was tested using air as the coolant for jet diameter Reynolds numbers from 4.65 × 10 2 to 1.405 x 10 3 . Heat transfer and pressure drop data were obtained for a range of mass rates extending up to the point of choked flow and also for variable heat fluxes. The results were compared to an existing Nusselt correlation for jet impingement arrays that was found to significantly under-predict the heat transfer. A new correlation is provided that also accounts for variable air properties

Axial steady free surface jet impinging over a flat disk with discrete heat sources

Abstract A free jet of high Prandtl number fluid impinging perpendicularly on a solid substrate of finite thickness containing small discrete heat sources on the opposite surface has been analyzed. Both solid and fluid regions have been modeled and solved as a conjugate problem. Equations for the conservation of mass, momentum, and energy were solved taking into account the transport processes at the solid–liquid and liquid–gas interfaces. The shape and location of the free surface (liquid–gas interface) was determined iteratively as a part of the solution process by satisfying the kinematic condition as well as the balance of normal and shear forces at this interface. The number of elements in the fluid and solid regions were determined from a systematic grid-independence study. A non-uniform grid distribution was used to adequately capture large variations near the solid–fluid interface. Computed results included velocity, temperature, and pressure distributions in the fluid, and the local and average heat transfer coefficients at the solid–fluid interface. Computations were carried out to investigate the influence of different operating parameters such as jet velocity, heat flux, plate thickness, and plate material. Numerical results were validated with available experimental data. It was found that the local heat transfer coefficient is maximum at the center of the disk and decreases gradually with radius as the flow moves downstream. The thickness of the disk as well as the location of discrete sources showed strong influence on the maximum temperature and the average heat transfer coefficient.

Test results of water and methanol high-speed rotating heat pipes

The existing rotating heat pipe (RHP) technology was examined and further developed for possible cooling applications in high-speed rotating electrical machines required for future designs of more-electric aircraft. Several technical papers covering the results of the design, fabrication, and testing of the RHP hardware in the low-to-intermediate rotational speeds from 500 to 7500 rpm have been published in recent years. In this investigation, performance results are presented for stainless steel 316-methanol and stainless-steel 316-water high-speed rotating heat pipes (HSRHPs) that were tested at speeds up to 30,000 rpm for the first time in the history of RHP research. The HSRHPs were cooled by air and oil jets in separate tests to compare their relative performances. Interesting temperature profiles and test results are presented for the HSRHPs that were heated by an induction heater and tested in the temperature range of 20-250°C for power inputs from 250 to 1300 W.

Active Cooling of Metal Oxide Semiconductor Controlled Thyristor Using Venturi Flow

A metal oxide semiconductor controlled thyristor (MCT) is a solid-state high-current switching device. Because of its high-current and high-heat dissipation, this device requires an advanced cooling arrangement. A sample MCT device was successfully tested in a conduction mode up to 95 A using a new technique called venturi cooling. Steady-state operational tests were performed under various coolant temperatures and flow rates. The highest device temperature was 168.5°C, whereas the power dissipation and heat flux were 170 W and 257 W/ cm2, respectively. Comparison with a commercial liquid-cooled cold plate showed that the cooling effectiveness is nearly double for the venturi flow. Measured junction-to-case thermal resistance of the MCT was 0.213°C/ W for venturi flow compared to 0.421°C/W for the commercial cold plate. Venturi flow cooling is highly recommended for MCT applications.

Finite difference simulation of transient heat pipe operation

An existing heat pipe numerical model has been modified in this study to better predict transient characteristics. Variable spacing in the axial direction has been incorporated into the nodal system to account for different evaporator, adiabatic section, and condenser lengths. Good agreement has been observed up to 350 K when compared to experimental data obtained from a refrigerant-11 heat pipe. The study shows that a finer grid in the evaporator (condenser) overpredicts (underpredicts) the operating temperature. The number of nodes in the adiabatic section appears not to affect the prediction significantly. Two different expressions for the effective thermal conductivity are also compared using the numerical model. 14 references.

Experimental investigation of oscillating heat pipes

Operation requirements of oscillating heat pipes (OHPs) are proposed. Based on the requirements, OHPs with nonflammable fluorocarbon fluids, FC-72 and FC-75, as the working fluid are developed. The OHPs have an inner diameter of 1.75 mm, a total length of 446 mm and 40 tubing turns. There is one evaporator in the middle and two condensers on both outer sides of the OHPs. Thermal performance tests are conducted at various operating temperatures and heat rates. The working fluid fill ratio is varied. A high performance OHP with FC-72 has been indicated for the first time. The FC-72 OHP can transport 2040 W heat rate without dryout. The gravitational acceleration does not have noticeable influence on the performance of the fluorocarbon OHP. The thermal performances of the fluorocarbon OHPs are compared with the case of an acetone OHP.