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International Journal of Heat and Mass Transfer 51 (2008) 5344–5358

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International Journal of Heat and Mass Transfer 51 (2008) 5344–5358 International Journal of Heat and Mass Transfer 51 (2008) 5344–5358 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt The application of plastic microcapillary films for fast transient micro-heat exchange Bart Hallmark, Christian H. Hornung, David Broady, Christopher Price-Kuehne, Malcolm R. Mackley * Department of Chemical Engineering, New Museums Site, Pembroke Street, Cambridge CB2 3RA, UK article info abstract Article history: This paper reports an experimental investigation into the heat transfer response of plastic microcapillary Received 20 June 2007 films (MCFs). The first section presents experimental determination of temperature profiles and heat Available online 29 May 2008 transfer coefficients for an MCF-based micro-heat exchanger. The next section presents two case studies: the first concerns cooling an electrical load, whereas the second examines heat transfer from an exother- Keywords: mic chemical reaction occurring within the MCF. The results demonstrate that heat transfer transients Plastic heat exchangers within MCF heat exchange devices are rapid. These systems provide an effective means of either heating Heat exchanger innovation or cooling fluids flowing within an MCF, or for removing heat from a hot surface. Micro-heat exchange 2008 Elsevier Ltd. All rights reserved. Microcapillary films Ó Thermal imaging 1. Introduction and background More generally, micro-scale heat exchangers have been the focus of intense study with the advent of manufacturing tech- There is currently interest in the field of micro-heat exchange niques such as photolithography, diamond machining and other and its application to a variety of heat transfer problems. One area micro-machining processes. Micro-heat exchange devices tend to that has received attention is the cooling of computer processors; be characterised by high volumetric heat transfer coefficients. A as the transistor density on these devices increases, and as their laminated, cross-flow, device fabricated from diamond-machined operating speeds accelerate, the thermal power dissipation rises. sheets of copper foil [7] attained volumetric heat transfer coef- The need to effectively and efficiently remove this waste heat is ficients of up to 44 MW/m3 K. Investigation was carried out on of great importance to the stability and long-term reliability of two different designs of wire-cut micro-heat exchangers, also these devices. An early investigation into on-chip liquid cooling fabricated from copper [8]. These devices consisted of either of integrated circuits recognised that a key heat transfer constraint empty deep channels or porous media filled channels and were was the heat transfer coefficient between the heated surface and able to achieve heat transfer coefficients of 38.4 and 86.3 MW/ the coolant flow [1]. Further research into on-chip heat exchange m3 K, respectively. Theoretical studies have also been under- includes the modelling and the creation of a design formulation taken to explore the most effective micro-channel shape when for on-chip liquid heat exchanger design [2] and various designs faced with problem of minimising the temperature of the hot of etched on-chip exchanger used with forced air cooling [3]. The surface [9], or when optimising the micro-channel geometry creation of on-chip liquid cooling systems using flow impingement with the constraints of high heat transfer and low pressure drop from micro-jets has also been studied [4] highlighting differences [10]. in the heat transfer process at the micro-scale when compared to Micro-heat exchange devices that take advantage of phase macro-scale jet impingement. Numerical and experimental study change on the cool side have also been investigated for cooling de- of the use of micro-channel arrays as heat spreaders to alleviate vices that exhibit very high heat fluxes. Water has been used in mi- on-chip hot spots has also been reported [5]. cro- and mini-channels to cool electron gun accelerator targets, In these studies, the heat exchangers have been fabricated on where heat fluxes in excess of 1 kW/cm2 can be found [11], the integrated circuit itself, using silicon as the substrate. Other whereas advantage has been taken of refrigerants to achieve phase studies have looked at alternative methods of cooling chips, using change in the coolant at lower temperatures [12]. non-silicon based materials. One study [6] concerned the charac- Small liquid hold ups, and large volumetric heat transfer coeffi- terisation of a polymeric heat exchanger, fabricated from poly-(di- cients, can lead to fast transient responses within micro-heat methyl siloxane) (PDMS), as an inter-chip heat exchange device for exchangers [13]. Fast transient heat transfer response makes these use in multiple chip packages, termed multi-chip modules. devices attractive for use in areas of science and engineering that fall outside of microelectronics cooling. One such application, * Corresponding author. DNA amplification, is carried out by a process termed the polymer- E-mail address: [email protected] (M.R. Mackley). ase chain reaction, or PCR [14], which involves the rapid thermal 0017-9310/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2008.01.036 B. Hallmark et al. / International Journal of Heat and Mass Transfer 51 (2008) 5344–5358 5345 Nomenclature 2 Acap total capillary surface area (m ) Greek letters 2 ALM log mean area (m ) d thickness (m) 2 AMCF surface area of one face of an MCF (m ) DTlm log mean temperature difference (K) hdi average capillary diameter (m) h heat transfer coefficient (W/m2 K) Subscripts I current (A) Al aluminium k thermal conductivity (W/m K) fluid fluid l MCF length (m) i measurement location n number of capillaries metal metal Qelectrical electrical power (W) PE polyethylene Qthermal thermal power (W) tape double sided tape Tfluid fluid temperature (K) Tmetal metal temperature (K) Non-dimensional group 3 Uv volume heat transfer coefficient (W/m K) Nu Nusselt number 2 Uoverall overall area heat transfer coefficient (W/m K) V voltage (V) cycling of a solution of DNA and enzymes between different tem- whilst performing heat transfer duties in a flow system. Water is perature regimes. pumped through an MCF wound onto a cartridge heater and the Fast and precise heating and cooling is also beneficial in many temperatures of the surface of the film are recorded in six different areas of chemical processing. Due to their small mass, microreac- locations. Using results from an additional experimental study, tors have very rapid thermal cycling times, and therefore the abil- presented in Appendix A, it is possible to relate the surface temper- ity to control secondary reactions and by-products by rapid cooling ature of the MCF to the average temperature of the fluid flowing in of a synthesised product after the reaction is complete, results in the MCF’s capillaries. Furthermore, thermal imaging is used to higher purities [15]. With the wide variety of possible applications investigate the natural convective cooling of hot water as is it is that require a fast heat transfer response on the micro-scale, cou- pumped along a vertically orientated MCF. Volumetric heat trans- pled with the typically laminar flow regime within the fluid flow fer coefficients are calculated for each case to serve as a compari- of these heat exchangers, this area of microengineering presents son with literature values [8]. exciting opportunities for the development of innovative materials The second section of this paper then presents two case studies and novel devices. where MCFs fabricated into heat transfer devices are evaluated. This paper is concerned with the transient heat exchange prop- The first of these presents an example where the direction of heat erties of a novel material termed microcapillary film (MCF). MCFs transfer is from a heated metal plate into fluid flowing within the are a class of novel, extrusion-processed, polymer films containing MCF, this providing a proof-of-principle for the cooling of elec- an array of continuous, parallel, capillaries that run along the film’s tronic components, such as computer processors. The second length [16,17]. The current embodiment of the MCF product con- example demonstrates the reverse scenario. A strongly exothermic tains 19 capillaries with a mean hydraulic diameter typically reaction is carried out within an MCF microreactor and the direc- around 200 lm; manipulation of process conditions can, however, tion of heat transfer is from the reacting mixture flowing within result in mean hydraulic diameters ranging between 30 and the MCF to the external environment. Thermal imaging was used 500 lm. MCFs have been successfully manufactured from a range in each of these two case studies, providing valuable insight into of different polymers; the polymer that was used for the MCFs in the nature of the heat transfer as well as providing quantitative this study was a commercially available linear low-density poly- information on thermal gradients and temperature distributions. ethylene (LLDPE) manufactured by the Dow Chemical Company All MCFs used in the experimental work in this paper contained Inc. (Dowlex NG5056G). A cross-section of a typical MCF is shown capillaries of average diameter 203 lm and standard deviation of in Fig. 1. 6.9%. A previous paper [18] presented an investigation into the hydraulic and steady-state heat transfer performance of LLDPE 2. The experimental determination of temperature gradients MCFs. This study demonstrated that, despite the plastic construc- within an MCF tion of the MCF, volumetric heat transfer coefficients of order 3 35 MW/m K could be attained using both sides of an MCF for heat 2.1. Fluid heating transfer, and around 14 MW/m3 K for single-sided MCF systems. Additionally, heat transfer in a variety of geometric configurations 2.1.1. Equipment and experimental protocol could be achieved due to the flexible nature of the MCF.
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