Effects of a Wire Mesh on Droplet Size and Velocity Distributions of Cryogenic Sprays

Cryogen spray cooling allows removing large amounts of heat in short periods of time. Previous investigations have shown that stainless steel wire meshes placed between spray nozzle and target surfaces effectively increase uniformity in lateral heat extraction while reducing cooling efficiency. In this study we used phase Doppler particle analysis to assess the effect of a wire mesh on the diameter and velocity radial distributions of cryogen droplets and, consequently, on the surface heat transfer. We measured at 40 mm downstream and various radial locations within steady state spray cones from two different nozzles without and with a mesh at two mesh-nozzle distances. Results show that with the mesh, droplets lose velocity and increase in size. The closer to the center of the spray cone the droplets are, the larger the loss in velocity. As a consequence, the radial velocity profile is more uniform relative to that without a mesh. NOMENCLATURE D mean drop diameter Do reference mean drop diameter [μm] d nozzle-tube inner diameter [mm] h mesh-nozzle distance [mm] N narrow nozzle r radial coordinate V mean velocity Vo reference mean velocity [m/s] V mean velocity difference W wide nozzle z surface-nozzle distance [mm] INTRODUCTION Cryogenic sprays have become an essential heat extraction technique in many industrial processes and medical procedures, such as electronic cooling and dermatologic laser surgery [1]. In general, uniform surface heat transfer is desired. However, because of the non-homogenous characteristics of sprays, such as droplet size and velocity, there are large lateral variations in heat transfer at the surface during spray cooling. For example, for applications that combine cryogenic spray cooling with laser heating, such as dermatologic laser surgery, non-uniform heat extraction may lead to excessive heating and potential irreversible skin damage at the laser beam periphery. Recent studies have demonstrated that lateral cooling on a flat Plexiglas surface is only uniform along a 2 mm radius within a 20 mm sprayed surface radius [2], and that passive control of mass deposition by means of a wire mesh significantly amplifies the radius of uniform cooling [3]. Wire meshes were first suggested in dermatologic laser surgery to enhance spray atomization and introduce a passive massdeposition control [4]. It was found that wire meshes reduce heat extraction efficiency and prolong cooling duration in the center of the sprayed area. Nevertheless, it is not clear how the mesh affects the spray and, thus, the surface heat transfer. The objective of the present study is to investigate lateral variations in the average droplet size and velocity distributions of vertical cryogen sprays due to the placement of a wire mesh at different heights. EXPERIMENTAL METHODS A scheme of the experimental set up is shown in Fig. 1. The spray injector and wire mesh were mounted on a BiSlide® positioning system (Velmex Inc., Bloomfield NY) to measure diameter and velocity from the center of the spray cone at 0 mm up to a 3 mm radius. Spray System Liquid cryogen R-134a, with a boiling temperature at atmospheric pressure of approximately -26 °C, is delivered to a fuel injector attached to a straight-tube nozzle. We used a 1.4 mm inner diameter (d) and 25 mm length nozzle, to be called 1 Copyright © 2005 by ASME wide (W), and a 0.7 mm inner diameter (d) and 25 mm length nozzle, to be called narrow (N). Fig. 1. Experimental set up Mass Deposition Control To control the deposition of liquid cryogen, a type 304 stainless steel wire mesh of size 400 (9231744 McMaster-Carr, Los Angeles CA) was placed between the laser volume probe and nozzle tip at h = 5 and 10 mm from the nozzle tip. The size of the mesh is based on the number of openings per inch of wire mesh. The diameter of the steel wire is 25 μm and the opening width is 38 μm. The mesh was allowed to dry before each spurt in order to avoid water condensation and freezing on the mesh which might have affected subsequent measurements. Phase Doppler Particle Analysis Phase Doppler anemometry allows measuring simultaneously the size and velocity of individual spherical particles or droplets with high spatial resolution. We used a phase Doppler particle analyzer (TSI Inc., Shoreview MN) to measure and compute the mean diameter and velocity of cryogen droplets at the center of the spray cone and every 1 mm along a 3 mm spray radius. Measurements were taken in steady state at z = 40 mm away from the nozzle. Cryogen sprays for medical applications are commonly applied 30–40 mm from the skin. To quantify the droplet size distribution we used the Sauter mean diameter which also gives information about the quality of atomization [5]. RESULTS AND DISCUSSION In Figs. 2–5: □, ○, and represent results without mesh and with mesh at 5 and 10 mm, respectively; and, horizontal bars on each symbol denote standard deviations. At least four independent measurements were taken at each location. Velocity (V) and diameter (D) are normalized respect to the mean velocity (Vo) and diameter (Do) at the center of the spray without mesh. It follows that V’ = V/Vo and D’ = D/Do; where Vo = 55.24 m/s and Do = 21.78 μm for W; and Vo = 44.42 m/s z