Impact of external temperature distribution on the convective mass flow rate in a vertical channel – A theoretical and experimental study

Abstract

Many works have been performed on heated vertical rectangular open-ended channels. While in most cases, thermal fields are quite well predicted or reproducible, there were often large unexplained variations in the experimental flow rate for apparently the same conditions. An experimental and theoretical investigation has therefore been carried out to identify the effect of external thermal stratifications on the flow rate. Four values of steady and uniform heat flux, equivalent to Rayleigh numbers between 2.9 × 10 4 and 1.6 × 10 8 were imposed experimentally either on one or both sides of a vertical rectangular channel, with various ambient thermal gradients external to the channel. It was observed that the mass flow rate was significantly reduced as the positive upward, external thermal gradient increased. A theoretical model of the phenomenon was also developed. There is an excellent agreement between the theoretically predicted and experimentally measured mass flow rates. This clearly highlights that external temperature distributions are key driving factors and their influence is accurately quantified in this work.

Detailed flow development and indicators of transition in a natural convection flow in a vertical channel

“…As was observed experimentally by Thebault et al [26], the ambient temperature, in the far-field was linear so that at any height in the surrounding ambient atmosphere the temperature is,…”

“…The experimental procedure used in each measurement session as well as the ambient room conditions have been described in details by Thebault et al [26].…”

“…For the great majority of the cases the room temperature distribution could be approximated by a linear distribution. In the numerical simulations, the stratified temperature distribution in the laboratory was therefore taken to be linear with a temperature gradient δ T (K/m) as done by Thebault et al [26].…”

“…The commercial software ANSYS FLUENT, in which the Vreman SGS model had been implemented as a user-defined function developed by Lau et al [23,24] was used to simulate the flow. For the two heat inputs investigated, an artificial inlet disturbance was used to mimic the environmental noise, inherent to experimental studies [2,8,19] It should be noted that, in recently published research [25,26,27], it has been demonstrated that the external thermal stratification is a confounding factor in natural convection studies in channels. For that reason it has been considered in the present work.…”

“…For that reason it has been considered in the present work. The experimental apparatus has already been described in Vareilles [5], Sanvicente et al [7] and Thebault et al [26] so that only the main features are presented here. The apparatus was located in a 6.6 m high, 4.6 m wide and 7.0 m long laboratory.…”