In-tube evaporation and condensation of HFC-134a and CFC-12 with various lubricant mixtures

Mandates currently in place by the Environmental Protection Agency require use of new environmentally acceptable refrigerants by 1996. Implementation of these new refrigerants by refrigeration and air-conditioning manufactures requires design data for the various components of these systems. This study focused on obtaining design data for evaporators and condensers of vapor compression systems. Specifically, a new environmentally acceptable refrigerant HFC-134a, which is targeted as a replacement for refrigerant CFC-12, was tested in an instrumented evaporator and condenser and their performance measured;Heat transfer coefficients and pressure drops were measured during evaporation and condensation of HFC-134a and CFC-12 in smooth and micro-fin tubes. Two different diameter smooth tubes and micro-fin tubes were tested: 9.52-mm outside diameter tubes and 12.7-mm outside diameter tubes. Micro-fin tubes are characterized by the numerous small fins that spiral down the inner surface of the tube. The micro-fin tubes used in this study had 60 fins with a spiral angle of 17° and fin heights of 0.2 mm. The heat transfer coefficients and pressure drops measured in this study were averages over the 3.66-m long test tubes. The refrigerant mass flux was varied from 125 kg/m[superscript]2·s to 375 kg/m[superscript]2·s in both the smooth and micro-fin tubes;The experimental data showed a significant increase in the performance of HFC-134a in the micro-fin tube as compared to the smooth tube. Specifically, evaporation heat transfer coefficients were increased by 50% to 100% with only a 10% to 30% increase in evaporation pressure drop, while condensation heat transfer coefficients were increased by 100% to 200% with a 40% to 100% increase in condensation pressure drop. Similar results were obtained for pure CFC-12 in the micro-fin tube;The effects of lubricant concentration (\prec5%) on the performance of HFC-134a and CFC-12 were also studied. HFC-134a was tested with a penta erythritol ester mixed-acid lubricant and a penta erythritol ester branched-acid lubricant. CFC-12 was tested with a naphthenic lubricant. Lubricant concentration in general decreased the heat transfer performance of refrigerants. The only exception was evaporation at low lubricant concentrations were the heat transfer coefficients were slightly enhanced by refrigerant/lubricant mixtures. Pressure drop during evaporation and condensation was increased by the addition of lubricant in most cases

An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12

Citation: Eckels, S.J. and M.B. Pate, An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12.International Journal of Refrigeration, 1991. 14(2): p. 70-77.Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m−2 s−1 for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation. Résumé On rapporte les coefficients de transfert de chaleur expérimentaux, pour le HFC-134a et le CFC-12, au cours d'un écoulement monophasique, à l'intérieur d'un double tube, et au cours de l'évaporation et de la condensation. Ces coefficients de transfert de chaleur ont été mesurés dans un tube lisse horizontal d'un diamètre intérieur de 8 mm et d'une longueur de 3,67 m. Le frigorigène dans le tube d'essai était chauffé ou refroidi par circulation d'eau dans un espace annulaire entourant le tube. Pour l'évaporation, les essais ont été effectués dans une plage de températures du frigorigène comprises entre 5 et 15°C, avec des qualités d'entrée et de sortie de 10 et 90% respectivement. Pour les essais de condensation, les températures du frigorigène étaient comprises entre 30 et 50°C, avec des qualités d'entrée et de sortie de 90 et 10% respectivement. Le flux massique a varié de 125 à 400 kg m−2 s−1 pour tous les essais. Pour des flux massiques similaires, les coefficients de transfert de chaleur d'évaporation et de condensation, pour le HFC-134a, étaient nettement supérieurs à ceux du CFC-12. Plus précisément, le coefficient de transfert de chaleur à l'évaporation du HFC-134a était de 35 à 45% supérieur à celui du CFC-12, et le coefficient de transfert de chaleur à la condensation supérieur de 25 à 35%

Contribution of wetted clothing to body energy exchange and heat stress

Citation: Elson, J., & Eckels, S. (2018). Contribution of wetted clothing to body energy exchange and heat stress. Journal of Thermal Biology, 78, 343–351. https://doi.org/10.1016/j.jtherbio.2018.09.014Quantifying the impact of clothing thermal and evaporation resistance is essential to providing representative boundary conditions for physiological modeling. In many models, sweat is assumed to drip off the skin surface to the environment and is not captured in clothing. In high metabolic rate and high temperature and humidity conditions the sweat produced by the body has the potential to saturate semipermeable clothing ensembles, changing the assumptions of the model. Workers, athletes and soldiers commonly wear encapsulating versions of such clothing to protect against environmental hazards. A saturated clothing model is proposed based on the ASHRAE two-node model using a saturated spot element in parallel with the existing method to account for sweat absorbed in the clothing. The work uses fundamental heat and mass transfer principles, modifying the existing formula using clothing measurements and basic assumptions. The effectiveness of the model is demonstrated by comparing the predictions of the original and proposed models, to the results of 21 soldiers exercising. The soldiers wore combat pants and shirt, helmet, gloves, shoes, socks, and underwear, and walked in a thermal chamber for 2 h at 42.2 °C dry bulb temperature, 54.4 °C wet bulb temperature, 20% relative humidity, and airspeed of 2 m/s. Core temperature, seven skin temperatures, heart rate, and total sweat loss were measured. The original model provides an average core temperature difference compared with the human subject results of 1.31 °C (SD = 0.557 °C) while the modified model improves the final prediction of core temperature to within an average of 0.15 °C (SD = 0.383 °C). The new model shows an improvement in the prediction of human core temperature under the tested conditions where dripping sweat will saturate clothing. The format can be used in multi-segmented thermal models and can continue to be developed and improved as more information on wetted clothing properties become available

Characterization of liquid refrigerant R-123 flow emerging from a flooded evaporator tube bundle

Citation: Asher, W., & Eckels, S. J. (2018). Characterization of liquid refrigerant R-123 flow emerging from a flooded evaporator tube bundle. Science and Technology for the Built Environment, 24(9), 1026–1038. https://doi.org/10.1080/23744731.2018.1464349The distribution of liquid droplets emerging from an evaporator tube bundle is characterized for low-pressure refrigerant R-123 with a triangular tube arrangement of pitch 1.167. The purpose of this research is to improve understanding of the droplet ejection process to aid the design of evaporators typically used in larger chiller systems. A laser and camera system captured images of the evaporator headspace at varying conditions. Conventional shadowgraphy techniques were applied to recognize droplets and match droplets for velocity calculations. The evaporator conditions were varied with mass fluxes from 3.5 to 40.7 kg/s-m2 (2550 to 30000 lb/hr-ft2), top rows heat fluxes from 5.3 to 31.5 kW/m2 (1700 to 10000 Btu/hr-ft2), and outlet saturation temperatures of 4.4°C and 12.8°C (40°F and 55°F). Conditions ranged from flooded to dry-out on the top rows. Droplet number, size distribution, and velocity are presented. The experimentaly measured liquid volume fraction in the headspace is also presented. Liquid distribution in the headspace is found to be a strong function of all varied properties, particularly mass flux, liquid level, and saturation temperature. The high liquid-vapor density ratio of R-123 and corresponding high velocities make it particularly difficult to separate liquid droplets before they escape the tube bundle

Multi-objective heat transfer optimization of 2D helical micro-fins using NSGA-II

Citation: Mann, G. W., & Eckels, S. (2019). Multi-objective heat transfer optimization of 2D helical micro-fins using NSGA-II. International Journal of Heat and Mass Transfer, 132, 1250–1261. https://doi.org/10.1016/j.ijheatmasstransfer.2018.12.078A numerical simulation of helical micro-fins is implemented in ANSYS Fluent 15. The model is scripted to automatically set up and execute given three input parameters: fin height, helix angle, and number of starts. The simulation results reasonably predict experimental pressure drop and heat transfer for multiple micro-fin geometries. A multi-objective parameter optimization is implemented based on the NSGA-II algorithm to estimate the optimal trade-off (Pareto front) between Nusselt number and friction factor of a micro-fin tube for 0.0006 < e/D < 0.045, 10 < Ns < 70, at Reynolds number of 49,013. The resulting Pareto front is analyzed and compared with several experimental data points. From the optimal results, a distinct difference in flow characteristics was identified between geometries above and below a helix angle of approximately 45°. How the Pareto front can be used to choose micro-fin geometries for different performance evaluation criterion scenarios is demonstrated. Optimal results from various existing correlations are also compared to the optimization results

Characterization and numerical simulation of liquid refrigerant R-134a flow emerging from a flooded evaporator tube bundle

Citation: Asher, W. E., & Eckels, S. J. (2019). Characterization and numerical simulation of liquid refrigerant R-134a flow emerging from a flooded evaporator tube bundle. International Journal of Refrigeration, 107, 275–287. https://doi.org/10.1016/j.ijrefrig.2019.07.001The distribution of liquid droplets emerging from an evaporator tube bundle is characterized for refrigerant R-134a with a triangular tube arrangement with a pitch of 1.167. The purpose of this research was to improve understanding of the droplet ejection process to aid in design of evaporators typically used in larger chiller systems. A laser and camera system captured images of the evaporator headspace at varying conditions. Conventional shadowgraphy techniques were applied to recognize and match droplets for velocity calculations. The evaporator conditions varied with bundle mass fluxes of 20.3 and 40.7 kg s−1m−2, top-rows heat fluxes of 15.8 and 31.5 kWm−2, and outlet saturation temperatures of 4.4 and 12.8 °C. Conditions ranged from flooded to dryout of the top rows. Droplet number, size distribution, velocity, and liquid volume fraction are presented in the headspace above the bundle. A method to numerically duplicate the droplet loading in the headspace using CFD with a Lagrangian discrete-phase model is also presented and verified, providing a powerful design tool. Liquid distribution in the headspace is found to be a strong function of all varied properties, particularly mass flux, liquid level, and saturation temperature

Analysis of particulate size distribution and concentrations from simulated jet engine bleed air incidents

Engine oil migrating into the bleed air stream of aircraft environmental control systems occurs with enough frequency and deleterious effects to generate significant public interest. While previous work has explored the chemical makeup of the contaminants in the aircraft cabin during these events, little is known about the characteristics of the aerosol resulting from oil contamination of bleed air. This paper presents particle counter data (giving both size distributions and concentration information) of the oil droplets from simulated jet engine bleed air. Four particle counters—a scanning mobility analyzer, an aerodynamic particle-sizer, an optical particle counter, and a water-based condensation particle counter—were used in the study encompassing a size range from 13nm to 20μm. The aerosol characterization is given for different bleed air temperatures and pressures. The data show a substantial increase of ultra-fine particles as the temperature is increased to the maximum temperatures expected during normal aircraft operation. This increase in ultra-fine particles is consistent with smoke generated from the oil. The pressure of the bleed air had little discernible effect on the particle-size and concentration

Relationship between turbulent structures and heat transfer in microfin enhanced surfaces using large eddy simulations and particle image velocimetry

Citation: Li, P., Campbell, M., Zhang, N., & Eckels, S. J. (2019). Relationship between turbulent structures and heat transfer in microfin enhanced surfaces using large eddy simulations and particle image velocimetry. International Journal of Heat and Mass Transfer, 136, 1282–1298. https://doi.org/10.1016/j.ijheatmasstransfer.2019.03.063Internally enhanced surfaces such as micro-fins are an important class of heat transfer enhancement in commercial applications. Many research papers discuss the design and optima of these surfaces. However, most previous studies have demonstrated only the macro relationship between the geometries of the micro-fins and heat transfer. The need for a deeper understanding of these fins arose from some currently unsolved problems that limit future development of enhanced surfaces. First, why are increases of heat transfer larger than area increases in micro-finned tubes in most cases? Second, why do internally micro-finned tubes typically have lower heat-transfer-enhanced ratios in laminar and transition flows? This work presents a novel method to analyze the detailed relationship between flow characteristics and heat transfer for one type of micro-fin. The goal of the paper was not to find a new Reynolds number-based correlation, but to find flow patterns responsible for heat transfer enhancement and understand the mechanisms that cause this. First, this paper introduces comprehensive experimental measurements including particle image velocimetry (PIV), measurement of the heat transfer coefficient and accuracy of pressure-drop measurements, all used to validate numerical approaches. Validated large eddy simulations (LES) are then used to predict flow characteristics and coherent structures (Q criterion). The numerical simulation includes both heat conduction in the metal structure and heat convection on the solid–fluid interface. Finally, the paper documents how the flow structures link with the enhancement of heat transfer in the micro-finned duct