R134a Spray Research Articles

Investigation of unsteady flow behavior of cryogen-spray coupled with cold air jet

Cryogen spray cooling (CSC) coupled with cold air jet (CAJ) was used to mitigate Cryogen consumption. However, the CAJ interaction could affect the flow structure and spray's unsteadiness, permitting coolant intermittency. In this study, the unsteady behavior and flow structure of CSC coupled with CAJ have been qualified. During the experiment, CAJ at an inclined angle of 30°, temperature of 266 K, flowrate of 0.0, 3.0, 6.0 m3/h, and axial distance of 20 mm from the spray-nozzle was injected into a typical R-134a spray system. A high-speed camera was used to capture the unsteady spray through the Mie-scattering technique, while large-eddy simulation (LES) coupled with discrete-phase model (DPM) was used to predict the vortical structure. The spatial correlation was used to identify the coherent structures, while the proper orthogonal decomposition (POD) was used to demonstrate their unsteady signatures. The results showed a significant impact of the CAJ on the spray-pattern and associated unsteadiness, providing an asymmetric spray-plume at the CAJ windward side. The CAJ increased the fluctuating energies of the unsteady coherent structures by 32 % through CAJ-to-spray interactions. It contributed to the unsteady spray through the counter-rotating vortex-pairs (CRVP) at the lateral sides and hairpin vortices along the injection line. The CRVP clustered the spray in recirculating regions, introducing unsteady CAJ entrainment at the leeward and thermal field at the windward. This work could insight into our understanding of unsteady multiphase flow, promoting advanced cooling technology by uncovering their causes and effects.

Experimental study on the heat transfer performance and dimensionless correlations for R134a spray cooling system

Spray cooling is a promising and effective cooling method, which may be used to solve the heat dissipation problem of high-power electronic components. In this paper, the influences of flow rate, sub-cooling degree, and refrigerant charge on the spray cooling heat transfer performance with R134a as the working fluid were experimentally studied. Results showed that the enhancement effect of the increase of sub-cooling degree on the heat transfer performance is very limited in this experiment. When the sub-cooling degree rises from 5 to 8 K, the heat transfer coefficient increases by 2.6% on average at lower heat flux (45.93–72.55 W/cm2). The increase of refrigerant charge can effectively improve the heat transfer performance, especially at higher heat flux (84.02–105.25 W/cm2). However, the Jacobian number decreases with the increase of the refrigerant charge, which hinders the improvement of heat transfer performance at higher heat flux. Therefore, there is an optimal refrigerant charge of 1.05 kg in this experiment. In addition, based on the experimental data, a Nusselt number dimensionless correlation of R134a solution with higher prediction accuracy was proposed.

New protocol for the R134acryogen spray cooling assisted 1064-nmlaser lipolysis.

Laser lipolysis is a promising body contouring technology. However, the skin tissue could be thermally damaged owing to the laser energy absorption by water, which limits the lipolysis efficiency. To protect skin tissue and improve the lipolysis effect, cryogen spray cooling is introduced and synergized with the pulsed laser irradiation aiming to propose a new therapy protocol. By simulating heat conduction in the skin after spray cooling assisted laser lipolysis, the temperature distribution in the skin tissue was obtained to analyze the tissue damage by the Arrhenius integral. After parameter optimization according to the damage threshold of skin and adipose tissue, a new protocol with high laser intensity and short time was proposed including 150-ms R134a spray cooling with spray distance of 30mm, and 100ms 1064nm laser irradiation with energy density of 20J/cm2, with a relaxation for 9.75s. This cycle of 10s can be repeated 90 to 150 times for a total of 15 to 25min. Compared with previous treatment procedure, new protocol can increase the fat dissolution depth from 2 to 4.5mm beneath the dermis with same order laser fluence.

Evaluation of the Performance of the Drag Force Model in Predicting Droplet Evaporation for R134a Single Droplet and Spray Characteristics for R134a Flashing Spray

Drag force plays an important role in determining the momentum, heat and mass transfer of droplets in a flashing spray. This paper conducts a comparative study to examine the performance of drag force models in predicting the evolution of droplet evaporation for R134a single droplet and spray characteristics for its flashing spray. The study starts from single moving R134a droplet vaporizing in atomispheric environment, to a fully turbulent, flashing spray caused by an accidental release of high-pressure R134a liquid in the form of a straight-tube nozzle, using in-house developed code and a modified sprayFoam solver in OpenFOAM, respectively. The effect of the nozzle diameter on the spray characteristics of R134a two-phase flashing spray is also examined. The results indicate that most of the drag force models have little effect on droplet evporation in both single isolated droplet modelling and fully two-phase flashing spray simulation. However, the Khan–Richardson model contributes to different results. In particular, it predicts a much different profile of the droplet diameter distribution and a much lower droplet temperature in the radial distance. The nozzle diameter has a significant impact on the flashing spray. A smaller diameter nozzle leads to more internse explosive atomization, shorter penetration distance, lower droplet diameter and velocity, and a faster temperature decrease.

Numerical Investigation of the Thermal Response to Skin Tissue during Laser Lipolysis

Laser lipolysis can effectively treat obesity and its associated diseases, such as hypertension, fatty liver, and hyperlipidemia. However, currently available invasive laser lipolysis, which transmits laser to a fiber-optic catheter inserted into the subcutaneous tissue for irradiation through an incision, may cause hematomas, infections, and empyrosis. The current study presents a novel, noninvasive approach for laser lipolysis, which directly irradiates the intact skin surface without an incision and preferentially targets adipose tissue at the near-infrared band. High laser energy is necessary to damage adipocytes; however, this may carbonate and burn the dermis. Therefore, the introduction of skin cooling is essential to avoid unwanted hyperthermal injury and improve the threshold of radiant exposure. In the current study, we investigated a novel noninvasive approach assisted with skin cooling by establishing a homogeneous multi-layer skin model. In this method, light propagation in the skin was simulated by using the Monte Carlo method. Skin cooling was employed before laser irradiation to protect the epidermis from thermal damage, which was treated as a boundary condition based on Newton’s law. The numerical results showed that the photons were deposited in the adipose layer more than in the other layers. Laser can effectively destroy adipose tissue at an energy density of >200 J/cm2 at 1210 nm wavelength, whereas at least 300 J/cm2 is required at 1064 nm to achieve the same effect. In this experiment, at >5 s pulse width, the selectivity of adipose was not obvious. Moreover, the results indicated that 60 ms R134a or R404a spray can effectively reduce the temperature of the epidermis. R404a exhibited a stronger cooling effect than R134a. Cold air cooling at −10 °C for 10 s could effectively decrease the skin temperature, and deeper cooling could be achieved by cold air cooling compared with cryogen spray cooling.

Parametric effect investigation on surface heat transfer performances during cryogen spray cooling

Cryogen spray cooling (CSC) has been a widely used auxiliary tool in laser dermatology such as port wine stain to prevent unspecific thermal injury due to laser energy absorption by the melanin in the epidermis. The present paper presents an experimental research on the effect of ambient temperature, relative humidity, and initial substrate temperature on heat transfer performances during R134a spray cooling. Results demonstrated that the cooling capability of R134a spray cooling can be obtained with small ambient temperature (Ta = 10 °C) and relative humidity (RH = 25%). Further investigation of cooling mechanism was conducted by studying the temporal and radial heat transfer distributions with different spray distances and nozzles. The heat transfer distribution presented large non-uniformity along radial locations. Two uniform cooling sub-regions of 0 ≤ r < 2 mm and 6 mm ≤ r < 10 mm were found under the spray distance of 30 mm and nozzle with an inner diameter of 1.0 mm. The heat transfer barrier was produced due to indirect contact between cold droplets and the substrate surface caused by bubbles and heat transfer is weakened by the low thermal conductivity of these bubbles.

Atomization and surface heat transfer characteristics of cryogen spray cooling with expansion-chambered nozzles

Cryogen spray cooling (CSC) is commonly applied in laser dermatology to protect the epidermis from thermal damage. Many efforts have attempted to improve the cooling capacity of CSC, among which the use of expansion-chambered nozzles is an effectively simple method with considerable potential. This study examined the influences of the expansion-chambered nozzle structure, including the ratios of inlet nozzle diameter to discharge nozzle diameter and of chamber diameter to discharge nozzle diameter on R134a and R404A spray cooling. Fifteen transparent expansion-chambered nozzles with the expansion chamber aspect ratio of 1.0, chamber diameter to discharge nozzle diameter ratios of 5.0–10.0, and inlet nozzle diameter to discharge nozzle diameter ratios of 0.6–1.4 were tested. The internal flow pattern inside the expansion chamber, external spray pattern, and surface heat transfer characteristics of cryogen spray using different nozzles, including the straight-tube nozzle, were investigated. The structure of the expansion chamber was found to have an important effect on the spray patterns and cooling characteristics. The spray radius obviously decreased when the expansion-chambered nozzles were used, and the spray pattern became narrower as the ratio of chamber diameter to discharge nozzle diameter increased. By contrast, the increase in ratio of two nozzle diameters enlarged the spray radius. Surface temperature and heat flux with different nozzles and cryogens showed a similarity, and correlations of surface temperature and heat flux were proposed. The introduction of expansion-chambered nozzles could effectively improve the spray cooling capacity. The minimum average surface temperature during the fully developed spray period could be reached for both R134a spray and R404A spray by an expansion-chambered nozzle with a chamber diameter to discharge nozzle diameter ratio of roughly 5.0 and an inlet nozzle diameter to discharge nozzle diameter ratio of roughly 0.6.

Numerical simulation of cryogen spray cooling by a three-dimensional hybrid vortex method

Cryogen spray cooling (CSC) can protect the epidermis from unwanted thermal damage during laser dermatological procedures. Numerical simulation of CSC can provide significant guidance for clinical implementation, but relevant research is sparse. Thus, a 3D two-way coupling hybrid vortex method is developed to simulate CSC. The two-way coupling and compressibility effects are implemented by introducing a vorticity source term into the grid. The validity of this algorithm has been confirmed in a previous simulation of the evaporating acetone spray. Comparisons between the simulation results and the R134a spray cooling experimental results demonstrate the accuracy of the present method in analyzing the complex atomization and evaporation. The simulated cooling capacity achieves the maximum experimental value at a spray distance of 26mm, which is close to the optimal spray distance of 30mm in clinical practice. Particulate count reveals that the central area with a radius of 2mm in R134a spray accounts for 74% of the total cooling capacity, which is beneficial for the precise control of the therapy area in laser surgery.

Comparative investigation on the spray characteristics and heat transfer dynamics of pulsed spray cooling with volatile cryogens

Cryogen spray cooling with R134a has been widely used in laser dermatology such as port wine stain to prevent unspecific thermal injury due to the absorption of laser energy by melanin in the epidermis. However, R134a spray cooling has shown poor efficacy for darkly-pigmented human skin. This paper presents an experimental study on the spray characteristics and heat transfer dynamics of pulsed spray cooling with three kinds of volatile cryogens (R134a, R407C and R404A), the latter two of which have potential to improve the cooling effect due to their lower saturation temperature. The spray patterns reveal that R404A generates the narrowest spray width and the smallest cooling spot on the cooling surface, which facilitates the accurate control of the cooling area. The R404A spray produces the smallest droplet diameter and the largest velocity, whereas the opposite applies to the R134a spray. The droplet temperature measurements show that R404A spray has the lowest temperature of spray, while the R134a spray has the highest. However, the non-dimensional temperature in the decrease period shows a similar pattern, independent of the cryogens. For the common spray distance in laser surgery (30mm), R404A spray leads to the lowest surface temperature and highest heat flux, and also removes the most heat from the cooling surface, indicating that R404A has the strongest cooling capacity. Furthermore, R404A spray has the least liquid film resistance time on the cooling surface, implying that it provides the best selectively spatial cooling effect. The results of spray and heat transfer all reveal that R404A possesses significant superiority with regard to pulsed spray cooling for epidermis protection, with the potential of substituting the current R134a in the application of laser dermatology, especially in the case of darkly-pigmented human skin.

Influence of chamber pressure on heat transfer characteristics of a closed loop R134-a spray cooling

A closed loop R134-a spray cooling system was designed using a commercial full-cone nozzle to generate the spray for the cooling of an upward plain copper surface. Given the flow rate, the spray cooling characteristics such as heat flux, critical heat flux (CHF) and spray efficiency were obtained under different spray chamber pressures. The results show the CHF and spray efficiency were enhanced with the increase of the chamber pressure. Due to the dependence of the saturation temperature on the vapor pressure, the cooling temperature of the target surface was directly proportional to the chamber pressure. Moreover, an improved empirical correlation was proposed for the non-dimensional heat flux as a function of the Jacob number, the Weber number, the dimensionless mass flow rate, the dimensionless pressure and the dimensionless bubble departure radius.

An experimental comparison of heat transfer characteristic between R134-a and R22 in spray cooling

Spray cooling is one of the promising technologies in heat removing for high power density equipment, especially for the equipment with high heat flux surface. R134-a is one of the most prospective alternative refrigerants of R22 due to the zero ODP, low GWP, non-toxicity and non-inflammability, it is necessary to do comparative study on the heat transfer characteristics of R22 and R134-a in spray cooling. In this article, two closed loop spray cooling test rigs with the same spray nozzle are established to investigate the thermal performance of R22 spray cooling and R134-a spray cooling. The main parameters such as critical heat flux (CHF), heat transfer coefficient and target surface temperature are compared in the same thermal condition. The CHF of R134-a spray is lower due to the lower latent heat. The heat transfer coefficient of R22 spray is higher than that of R134-a spray with the same spray chamber pressure. However, R134-a spray cooling could replace R22 spray cooling in the phase change heat transfer region when the heat flux is less than 80Wcm−2.

Experimental study on the characteristics of a closed loop R134-a spray cooling

Experimental investigation on the R134-a spray cooling characteristics is presented in a closed loop system. A full-cone nozzle was applied to spray onto a heated circular copper plate. The characteristics of the R134-a spray cooling under a series of different volumetric flow rate were obtained. The results shown that volumetric flow rate improves the critical heat flux (CHF). And larger volumetric flow rate prevent the dry-out phenomenon under high heat flux. However, volumetric flow rate bring the adverse condition that reduces the system efficiency. The maximum CHF of 117.2Wcm−2 was achieved with 319K target surface temperature at 0.356Lmin−1. The results indicate that R134-a owns a great cooling characteristics in spray cooling system.

Investigation on heat transfer characteristics of R134a spray cooling

Due to its high local heat transfer coefficients, spray cooling is a vital technique for thermal management of high heat flux electronic devices. In this study, an experimental system using R134a as the working fluid was designed and built to study the heat transfer characteristic of spray cooling. The influences of the liquid flow rate, the vapor phase mass fraction and the liquid subcooling of the two phase R134a spray cooling on the heat transfer performances were discussed. The experimental results show that the heat transfer coefficient increased with the increasing liquid flow rate when the liquid fluid rate was less than 1.6L/min. However, the enhancement on heat transfer coefficient owing to the increase of liquid flow rate was insignificant while the flow rate was higher than 1.6L/min. A maximum surface heat flux of 325.99W/cm2 was achieved using R134a spray cooling while the temperature of the test surface was about 42.4°C at the flow rate of 2.15L/min.

R-134a spray dynamics and impingement cooling in the non-boiling regime

R-134a spray as it impinges on the flat endplate of a circle is studied experimentally. In order to optimize R-134a spray cooling efficiency, a detailed characterization and understanding of liquid spray formation is essentially needed. An optical image system was used to quantify the spray flow structure. LDV measurements were used to characterize the local velocity /and velocity fluctuation distribution from a commercial available nozzle in both axial and radial directions. The radial velocity are found to be the largest at the outer edges of the spray, and they continuously decrease across the spray toward the center axis; while the corresponding axial velocity is the maximum there. Moreover, spray heat transfer in non-boiling regime was shown to be dependent on the velocity of the impinging spray in terms of Weber number and other related parameters which are in good agreement with those of previous studies.