Latent heat

About: Latent heat is a research topic. Over the lifetime, 13503 publications have been published within this topic receiving 302811 citations.

Flow condensation heat transfer coefficients of pure refrigerants

Abstract: Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m −2 s −1 and heat flux of 7.3–7.7 kW m −2 . Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.

Surface Energy Budget at Amdo on the Tibetan Plateau using GAME/Tibet IOP98 Data.

Abstract: In the east of Tibetan Plateau, Amdo, the surface energy flux measurement with eddy correlation technique was conducted during the Intensive Observation Period (IOP) of GEWEX Asian Monsoon Experiment (GAME). This study is a preliminary analysis on surface energy budget at Amdo with these data. There is a remarkable change in sensible and latent heat fluxes between pre-monsoon season and summer monsoon season. In the pre-monsoon, the ground surface and the surface layer were very dry. The specific humidity was 2 to 4 g/kg and the sensible heat flux was dominant. As the monsoon progresses, the surface becomes wet due to almost daily precipitation. Accordingly, the sensible heat flux decreases and the latent heat flux increases. In the beginning of September, nearly the end of the summer monsoon, the latent heat flux exceeds the sensible heat flux. The ground heat flux at the surface was estimated using the observed soil temperature profile with the aid of the thermal conductive equation. The computed ground heat flux corresponds to the residual of the surface energy balance in the daytime. But disagreement occurred in the evening, when the surface temperature rapidly decreases. On the daily averaged bases, the surface energy balance is not well closed. In terms of closure ratio, the value in a typical clear day was 67%. Another rough estimation of soil heat flux is made, in which the heat needed to melt the soil water and to heat up the soil layer was calculated. According to this, it was estimated that about 30 W/m2 (20% of net radiation) of soil heat flux was required on average over April 20 to July 20, but the measured soil heat flux was only 5.7 W/m2.

Summertime surface energy budget and ablation modeling in the ablation zone of a maritime Tibetan glacier

Abstract: [1] The surface energy budget and ablation were measured in the ablation zone of Parlung No. 4 Glacier on the southeast Tibetan Plateau (29°14′N, 96°55′E) during boreal summer 2009. The present study examines the summertime surface energy fluxes to identify major atmospheric variables governing the surface melt and their phenomenological links to the progression of the South Asian monsoon. Turbulent sensible heat and latent heat fluxes were calculated using the bulk aerodynamic approach, the accuracy of which was verified through a comparison with eddy-covariance flux measurements. The surface ablation calculated by the energy balance model was also verified by measurements of ablation stakes. Our results found the following percentage contributions to the total melt energy: net shortwave radiation, 98%; net longwave radiation, −12%; sensible heat, 16%; latent heat, −1%; and subsurface fluxes, −1%. The combined roles of cloud cover and surface albedo appear to control the surface energy balance during the onset period of the South Asian monsoon. The cloud variations affect surface melting with the advancement of the monsoon. Intensification of the South Asian monsoon probably accelerates melting in the ablation zone, whereas weakening of the monsoon reduces glacier melting, mainly because of changes in downward longwave irradiance and heat release due to vapor condensation. Moreover, the temperature index model proves useful for long-term mass balance and ablation modeling in cases where the degree-day factors are calibrated. But incorporating incoming shortwave radiation into the model should be more applicable and practical in this region.

Improved Wind Measurements on Research Aircraft

Abstract: Improved techniques for measuring horizontal and vertical wind components and state variables on research aircraft are presented. They include a filtering method for correcting ground speed and position Inertial Navigation System data with Global Positioning System data, use of moist-air thermodynamic properties in the true airspeed calculation, postflight calculation of the aircraft vertical velocity, and calibration of airflow attack and sideslip angles from the two air-data systems on each aircraft—a radome gust probe and a pair of fuselage-mounted Rosemount 858Y probes. Winds from the two air-data systems are compared for the National Oceanic and Atmospheric Administration WP-3D aircraft. As an evaluation of these techniques, data from the two aircraft during side-by-side low-level constant-heading runs are compared for mean and turbulent measurements of wind, ambient temperature, and absolute humidity. Small empirical offsets were determined and applied to the two latter scalars as well as to static and dynamic pressures. Median differences between mean horizontal wind components from nine comparisons were within 0.1 6 0.4 ms 21. Median differences in latent heat and sensible heat fluxes and momentum flux components were 3.5 6 15 Wm 22 ,0 6 2.5 Wm 22, and 0 6 0.015 Pa, respectively.