M.K. Mittal

An Experimental Study of the Flow of R-407C in an Adiabatic Spiral Capillary Tube

The objective of this study is to investigate the effect of coiling on the flow characteristics of R-407C in an adiabatic spiral capillary tube. The characteristic coiling parameter for a spiral capillary tube is the coil pitch; hence, the effect of the coil pitch on the mass flow rate of R-407C was studied on several capillary tube test sections. It was observed that the coiling of the capillary tube significantly reduced the mass flow rate of R-407C in the adiabatic spiral capillary tube. In order to quantify the effect of coiling, the experiments were also conducted for straight a capillary tube, and it was observed that the coiling of the capillary tube reduced the mass flow rate in the spiral tube in the range of 9–18% as compared with that in the straight capillary tube. A generalized nondimensional correlation for the prediction of the mass flow rates of various refrigerants was developed for the straight capillary tube on the basis of the experimental data of R-407C of the present study, and the data of R-134a, R-22, and R-410A measured by other researchers. Additionally, a refrigerant-specific correlation for the spiral capillary was also proposed on the basis of the experimental data of R-407C of the present study.

An investigation of double-glass-covered trapezoidal salt-gradient solar pond coupled with reflector

ABSTRACT In the present study, a trapezoidal salt-gradient solar pond (TSGSP) has been investigated experimentally. The top surface of solar pond has been covered with double-glass cover in order to reduce the evaporative and convective losses from the top. This results in increase of temperature even in the top zone of the solar pond and leads to more volume utilization for heat storage in the pond. A reflector made of aluminium sheet has been used to enhance the solar intensity on the solar pond during sunny hours. A procedure, to determine optimum tilt angle of reflector in order to utilize maximum amount of solar energy at noon, has been proposed. The use of reflector enhanced the average solar intensity on the top surface of solar pond by 22%. The maximum average temperature of trapezoidal solar pond with glass cover and reflector has been observed to be 70.5°C. The thermal efficiencies of LCZ, NCZ and UCZ for the trapezoidal solar pond with double-glass cover and reflector have been estimated to be 32.73%, 23.22% and 5.30%, respectively. In addition to experimental investigation, the sunny area ratio of TSGSP has been theoretically computed and compared with the cuboid solar pond having same top surface area and depth in order to see the effect of pond shape on sunny area ratio. The average yearly sunny area ratio of trapezoidal solar pond has been determined to be 11% higher than that of cuboid one.

Applicability of Heat Mirrors in Reducing Thermal Losses in Concentrating Solar Collectors

Flux distribution around the parabolic trough receiver being typically non-uniform, only a certain portion of the receiver circumference receives the concentrated solar irradiance. However, radiative and convective losses occur across the entire receiver circumference. This paper attempts to introduce the idea employing transparent heat mirror to effectively reduce the heat loss area and thus improve the thermal efficiency of the solar collector. Transparent heat mirror essentially has high transmissivity in the solar irradiance wavelength band and high reflectivity in the mid-infrared region thus it allows the solar irradiance to pass through but reflects the infrared radiation back to the solar selective metal tube. Practically, this could be realized if certain portion of the conventional low iron glass envelope is coated with Sn-In2O3 so that its acts as a heat mirror.In the present study, a parabolic receiver design employing the aforesaid concept has been proposed. Detailed heat transfer model has been formulated. The results of the model were compared with the experimental results of conventional concentrating parabolic trough solar collectors in the literature. It was observed that while maintaining the same external conditions (such as ambient/initial temperatures, wind speed, solar insolation, flow rate, concentration ratio etc.) the heat mirror-based parabolic trough concentrating solar collector has about 3–12% higher thermal efficiency as compared to the conventional parabolic solar collector. Furthermore, steady state heat transfer analysis reveals that depending on the solar flux distribution there is an optimum circumferential angle (θ = θoptimum, where θ is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses.Copyright