Promotion of Turbulent Thermal Mixing of Hot and Cold Airflows in T-junction

Abstract

An experimental study has been conducted on the promotion and control of turbulent thermal mixing of hot and cold airflows in a T-junction with rectangular cross sections, which simulates the HVAC unit for automobile air-conditioning system. In order to promote the turbulent thermal mixing, small jets have been blown into the main channel at the upstream edge of the T-junction in the direction of 45°against the main flow. Turbulence intensity in the upper part of the thermal mixing layer can be increased with these jets, and consequently the turbulent mixing of hot and cold airflows is promoted effectively. Moreover, it has been found that the degree of thermal mixing can be controlled by changing the jet velocity.

Large Eddy Simulation of Turbulent Flow in a T-Junction

“…According to Hirota et al (2008), the velocity distributions measured under the isothermal condition agreed with those obtained with T h =60 o C and T c =12 o C, suggesting that the buoyancy effect on the mixing is negligibly small. In this study, the temperature difference is 17 o C so the present treatment of the temperature is reasonable.…”

Promotion and control of turbulent mixing of hot and cold airflows in T-junction

“…In this channel geometry, the heat transfer between the hot and cold airflows is dominated by the turbulent heat flux vt and its main production term is expressed as v 2 T/Y. Therefore, the production of vt is quite small in the upper part of the thermal mixing layer (Hirota, et al, 2008) and this causes such a limited development of the thermal mixing layer in the upward direction as observed in Fig. 6.…”

“…The specification of this channel was essentially the same as that used in our preceding study (Hirota, et al, 2010) except that the second edge (edge in the downstream side) of the T-junction was planed off. We chamfered this second edge because high turbulence caused by the flow separation at this sharp edge was located below the thermal mixing layer, and did not contribute so much to the turbulent thermal mixing of two flows (Hirota, et al, 2008). By chamfering this edge, the scale of the separation bubble was considerably reduced.…”

“…Therefore, the promotion and control of turbulent mixing of hot and cold airflows are key subjects in the development of the HVAC unit. Active methods that need external power inputs, such as small jets, are quite effective to the mixing promotion/control of two flows (Bonnet, et al, 2008, Hirota, et al, 2008, but the passive methods without any external power inputs are required in the practical application to the HVAC unit for the automobile air-conditioning system. With these points as background, we conducted an experimental study on the promotion and control of turbulent mixing of hot and cold airflows in the T-junction with rectangular cross sections.…”

Investigation on the thermal mixing enhancement in a T-junction pipe

“…Various authors have investigated flows in T-junctions experimentally in the past. Hirota et al [5] investigated the improvement in thermal mixing of hot and cold fluid flows in a T-junction. They concluded that the turbulence in the upper part of the mixing layer could be improved by blowing the jets through the main pipe at the upstream edge of the T-junction.…”