Effects of Ambient Pressure on R134a Sprays for Laser Dermatological Applications

R134a sprays have been widely used as a method to protect the epidermis during laser therapies of hyper-vascular lesions, such as port wine stains, due to their high heat flux and precise control of cooling duration. Recent studies have shown that the use of vacuum cups during laser treatment can physically dilate the vasculature of interest and improve therapeutic outcome. However, the effects of these vacuum pressures, and the corresponding changes in humidity level, on the cooling sprays remain unknown. In this paper, characteristics of steady-state R134a sprays, measured with a Phase Doppler Particle Analyzer (PDPA) at different humidity levels and pressures are presented. Results show high non-uniformity in droplet characteristics across the spray cross-section at atmospheric pressures. With increasing humidity from 15 % to 75 %, no measurable changes in droplet diameter or velocity are evident, though spray cone diameter and penetration increase. Ambient pressure changes from 0 to -55 kPa, however, result in significant droplet size reductions while simultaneously increasing droplet velocities. These changes appear to occur mainly during primary atomization of the spray. The resulting implications for spray atomization and cooling effectiveness are also discussed. Corresponding author Introduction Laser treatment of various hypervascular dermatoses, particularly port wine stains, has become common practice in recent years [1, 2]. Laser light of an appropriate wavelength (585 nm) is used because it is highly absorbed by the target chromophore, hemoglobin, within the vasculature. This absorption of energy induces the desired thermal necrosis of the hypervascular lesions. However, melanin within the epidermis also absorbs a wide spectrum of light energy. To avoid epidermal injury by heating, a method of precooling using a short duration refrigerant spray has found widespread acceptance because of the possible high heat fluxes and precise control of cooling duration [3]. Thus, rapid and spatially selective cooling of the epidermis is possible without lowering the temperature of the deeper-seated target chromophores. With precooling, the epidermis is kept below the damage threshold during heating of the tissue by the subsequent laser pulse. Despite the effectiveness of this epidermal protection technique, complete blanching of the lesions is rarely achieved. Darker-skinned patients also normally cannot be treated. Treatment effectiveness could potentially be improved by increasing laser fluence, but this is limited by the epidermal protection that the sprays can provide. Additionally, the sprays have been found to induce highly non-uniform cooling [4], potentially leading to uneven protection and skin dyspigmentation or scarring. Many studies have been done to characterize these refrigerant sprays and their accompanying heat transfer [5-8], but the mechanisms of atomization and heat transfer are still not well understood. Recently, a new treatment technique using vacuum suction cups to dilate the blood vessels and increase blood volume fraction prior to laser treatment has shown promising results [9, 10]. Improved blanching with the same radiant exposure is possible. As air is evacuated from these suction cups, both air pressure and humidity are reduced. In general, humidity can also vary in the environment without the use of the suction cups. The effects of hypobaric pressures and changing humidity on the spray itself have not been quantitatively identified. Therefore, the purpose of this study is to quantitatively characterize the spray and observe the changes due to varying ambient pressure and humidity. Explanations for the observed changes will be given, along with implications on heat transfer. Experimental Methods Spray system The liquid refrigerant used was R-134a (1,1,1,2tetrafluoroethane; National Refrigerants, Philadelphia, PA) which was maintained at saturation pressure 660 kPa at room temperature) in a standard 30 lb bottle. Figure 1. A schematic of the PDPA system and clear acrylic chamber. The refrigerant was delivered via a high pressure hose to a clinically-used solenoid valve and nozzle (GentleLASE; Candela, Wayland, MA). The nozzle was a stainless steel plain circular orifice tube of 0.5 mm inner diameter and approximately 25 mm length. There was a 20° bend in the nozzle near the solenoid valve body, but the nozzle exit was oriented to deliver horizontal sprays. Axial positions, z, were defined as the vertical distances between the nozzle exit tip and the probe measurement volume. The solenoid valve and nozzle were displaced using a commercial translational axis (Pittman Lo-Cog, Harveysville, PA) to desired z spanning 15 to 90 mm with 15 mm increments. Errors in z were approximately ±1 mm. In clinical practice, z is normally fixed at 30 mm so this study includes the clinically relevant distance. Spray characterization Spray droplet velocity and diameter were measured using a Phase Doppler Particle Analyzer (PDPA; TSI Incorporated, Shoreview, MN) with an Argon ion laser emitting 488 and 514.5 nm wavelength light beams. A schematic of this setup is shown in Figure 1. This system was capable of measuring velocity along two perpendicular axes, but only the axial velocities will be presented since the magnitudes of the lateral (radial) velocities were significantly smaller. All presented PDPA diameter and velocity values were averages of a minimum of 10000 measurement points taken from steady-state sprays. According to Tate and Marshall [11], this corresponds to an error in cumulative distribution of less than 1.5%. D10 averaging of diameter was used because it is a first-order expression and, therefore, less sensitive to measurement errors. Ambient condition control In order to control the ambient pressure and humidity of the environment surrounding the nozzle, a custom Spray nozzle measurement volume photodetector laser 120° atmosphere Clear acrylic chamber