Search

Thermal design of tubular heat exchanger based on condensation heat of water steam requires knowledge of condensation heat transfer in the each tube. This paper is just aimed on experimental analysis of steam condensation in vertical copper tube in length of 1285 mm with 2.0 mm inner diameter and 0.5 mm wall thickness. Experimental measurement is performed in 12 steps with variable inlet temperature and mass flow rate of water steam. The heat transfer coefficient on the inner surface of tube in condensation zone is calculated by Thermal resistance method and Wilson plot method. The correlation quality of results obtained from both methods is 98.8%. The results are compared with other experimental studies and also correlated with five chosen equations for prediction of condensation heat transfer coefficient. The correlation quality of results obtained from this experimental analysis and four tested equations is over 96.6%, only theoretically determined Nusselt equation undervalues condensation heat transfer coefficient as is known. The Nusselt equation does not take to account waves on condensate surface. These waves on condensate surface are caused by flow of water steam in tube and the wave’s effect is approximately 20.5% in this presented case. [ABSTRACT FROM AUTHOR]

Light pipes transmit daylight into building interiors. Their installation into thermally insulated roofs of low energy buildings can be a problem because of thermal bridges and condensation problems. This article is focused on a CFD simulation thermal analysis that includes four variations of light pipes with a segment of a flat roof. Common light pipes with a hollow light guiding tube were compared to special light pipes containing an additional glass unit located inside the tube. The additional glass units increase thermal resistance and reduce condensation risks of the light guiding systems. The light pipes were compared in two different simulation models run in ANSYS Fluent software and the CalA program. Temperature profiles and air flow patterns of the cross sectional profiles of the light pipes served to determine the total heat transmittance and heat losses of the studied light pipes installed in a segment of a thermally insulated flat roof. The paper compares simplified 2D rotational–symmetrical numerical model based on the thermal diffusion equation with the complex 3D CFD numerical simulation. The results confirm that the simplified 2D numerical model is suitable for the thermal evaluation of the light pipes containing an additional glass unit, too. The additional glass unit with the triple glass improves thermal resistance up to 88% in case of light pipe with diameter 600 mm and reduces optical transmittance to 28%. [ABSTRACT FROM AUTHOR]

Tubular light guides are applicable for daylighting of windowless areas in buildings. Despite their many positive indoor climate aspects they can also present some problems with heat losses and condensation. A computer CFD model focused on the evaluation of temperature distribution and air flow inside tubular light guides of different dimensions was studied. The physical model of the tested light guides of lengths more than 0.60 m proves shows that Rayleigh numbers are adequate for a turbulent air flow. The turbulent model was applied despite the small heat flux differences between the turbulent and laminar model. The CFD simulations resulted into conclusions that the growing ratio of length/diameter increases the heat transmission loss/linear transmittance as much as by 50 percent. Tubular light guides of smaller diameters have lower heat transmission losses compared to the wider ones of the same lengths with the same outdoor temperature being taken into account. The simulation results confirmed the thermal bridge effect of the tubular light guide tube inside the insulated flat roof details. The thermal transmittance of the studied light guides in the whole roof area was substituted with the point thermal bridges. This substitution gives possibility for simple thermal evaluation of the tubular light pipes in roof constructions. [ABSTRACT FROM AUTHOR]