Mark North

Bio: Mark North is an academic researcher. The author has contributed to research in topics: Heat sink & Heat transfer. The author has an hindex of 11, co-authored 28 publications receiving 579 citations.

Heat and Mass Transport in Heat Pipe Wick Structures

Abstract: temperature field,areobtainedfortheporouswicksundertheactionofadiscreteheatsource(evaporator)mounted on one end. The working fluid, supplied from a condenser pool, evaporates from the wick surface primarily in the evaporator region and is condensed and collected into a container separate from the pool, to yield mass flow rates. Thus the liquid-pumping capability of the wick, coupled with flow impedance, is measured as a function of applied heat flux.Repeatableresultswithlowuncertaintyareobtained.Acarefulanalysisofthetransportpathsforheatand masstransferin thewickstructure confirmsthatmasstransfer duetovaporization oftheworking fluidisthelargest contributor to heat dissipation from the wick. The expected and measured values of evaporation rate are in good agreement. Results are also presented in terms of overall effective conductance based on measured temperatures.

Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications

Abstract: Passive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K-333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm-0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm2 , while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs 50 (>;W/cm2).

High frequency, large displacement, and low power consumption piezoelectric translational actuator based on an oval loop shell

Abstract: Compact piezoelectric actuators based on an oval loop shell structure were fabricated and their vibration characteristics were investigated. The actuators can successfully create a translational motion at a high frequency and a large displacement working distance at the second resonant mode of the shell structure. As a result of a shot peening process to harden the surface and increase the strength of the shell structures, fatigue limits were enhanced. The highest operating frequency of 1444 Hz was achieved with about 1.5 mm translational displacement under an applied voltage of 100 VAC. The largest amplified displacement of 2.1 mm was obtained at a resonant frequency of 961 Hz. Displacement amplification ratios between static and resonance conditions are presented and compared. A theoretical approach was provided to estimate the natural frequencies of the oval loop shell actuators. The estimated natural frequencies of the actuators agreed with experimental values to within 12%. In addition, load bearing capacity and efficiency of one of the shell actuators was evaluated with an experimental method. The calculated actuator efficiency is around 55% when 3.1 g of mass is loaded to the actuator and an applied voltage of 140 V is applied. A possible application of the actuator, a cooling device, was demonstrated by providing its configuration and test results.

Heat and Mass Transfer

Abstract: Thermophysical Properties Research Literature Retrieval Guide Edited by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore Second edition, revised and expanded. Book 1: Pp. xxi + 819. Book 2: Pp.621. Book 3: Pp. ix + 1315. (New York: Plenum Press, 1967.) n.p.

Do surfaces with mixed hydrophilic and hydrophobic areas enhance pool boiling

Abstract: We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7° wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are, up to respectively, 65% and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer. © 2010 American Institute of Physics. doi:10.1063/1.3485057 Boiling is an efficient process to transfer large amounts of heat at a prescribed temperature because of the large latent heat of vaporization. The term flow boiling describes the boiling of liquids forced to move along hot surfaces, while in pool boiling, the topic handled in this paper, the liquid is stagnant and in contact with a hot solid surface. 1 Besides the common experience of boiling water in an electric kettle, pool boiling has applications in metallurgy, high performance heat exchangers, and immersion cooling of electronics. Pool boiling performance is measured with two parameters, the heat transfer coefficient HTC and the critical heat flux CHF. The CHF is measured by increasing the surface temperature until a transition from high HTC to very low HTC occurs. This signifies the formation of a vapor film insulating the liquid from the heated surface, a phenomenon called dry out. Several characteristics determine the performance of a boiling surface. Nucleation sites in appropriate number and dimensions need to be provided such as cavities, rough areas, or hydrophobic islands. 2 As of today, the performance of boiling surfaces has been increased by using wicking structures to prevent dry out, 3 by increasing the surface area with fins or fluidized bed, 3‐6 and by enhancing the wettability of the surface. 5‐10 The latter strategy is justified by experiments of Wang and Dhir, 11 showing that the CHF was

Surface engineering for phase change heat transfer: A review

Abstract: Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries.