High Heat Flux Cooling by Liquid Jet-Array Modules

Heat removal at high power density is a continually challenging problem. Traditionally, the highest heat fluxes have been removed by flow boiling systems, for which critical heat flux is the usual limiting factor. Indeed, the highest steady-state heat fluxes obtained have generally been CHF values for tube flows of water, with maximum critical fluxes in the range of 100 to 350 MW/m. These heat loads can generally be supported over only a small length of the tube. Recently, jet impingement systems have been investigated as alternatives to tube flow systems for the highest heat fluxes. Liu and Lienhard [1] used high-velocity water jets to cool a small heated region, with fluxes ranging between 50 and 400 MW/m. They confined the heating to the high-pressure stagnation region of a 1.9 mm diameter jet. Jet velocities ranged from 50 to 134 m/s, and associated stagnation pressures were between 12 and 89 atm. Their heat fluxes included the highest steady-state fluxes reported to date. The fluxes obtained were generally limited by the heat source used or by the ability of the heat transfer surface to support large thermal stresses, and the data showed no evidence of critical heat flux limitations. In either of the above situations, the heat loads were supported over only a small area, but in many circumstances it is desirable to carry large fluxes over larger areas. In this note, we report a cooling module designed to carry heat fluxes of up to 20 MW/m over areas of 10 cm or more [2,3]. The module uses an array of impinging liquid jets to convectively cool the rear of a faceplate whose forward surface is subjected to an imposed heat load. Experimental studies of very high heat fluxes can often be limited by the availability of a suitable heat source, particularly when it is desired to impose a fixed heat flux upon a solid surface. Such solid surfaces usually represent the main surface of a cooling system and separate the cooling fluid from the environment that delivers the heat load. In practice, these large heat fluxes arise when plasmas, lasers, or X-rays interact with the cooling surfaces. In the case of plasma fusion systems, fluxes have design levels near 5 to 30 MW/m over areas of several square meters, and the transferred heat is used to drive power conversion cycles [4]. In the case of synchrotron X-ray monochrometers, the fluxes may exceed 90 MW/m over areas of square millimeters and represent undesired heat loads that must be removed to maintain system performance [5]. Large carbon dioxide lasers, electron beams, plasma arcs, and linear accelerators have all been used to deliver heat fluxes in this range for experimental testing [1,6,7], but such methods are either very expensiveor produce spatially nonuniform heat loads (generally Gaussian distributed) that may not represent actual operating conditions. An alternative method is to form an electrical resistance heater atop the test surface [8,9]. In the present work, we have deposited a thin metal film onto the test surface using thermal spraying; this film serves as a resistance heating element that can provide a uniform and easily controlled flux. A thin dielectric layer is sprayed first, so as to isolate the heating element from the metallic faceplate surface. High current, low voltage DC power is used to drive the heater. The direct deposition of the films onto the test surface, as opposed to a mechanically attached heater, helps to minimize interfacial contact resistances which can cause large temperature differences at these high flux levels.