Air curtains: validation of results from small-scale laboratory waterbath experiments by real-scale measurements in climate chambers and numerical simulations

Abstract

An air curtain is a planar turbulent air jet, which is used to separate two zones maintained at different temperatures. One of the aims of using an air curtain in the doorway of a building is to minimise the heat losses from the warm room to the cold environment and thus, to save energy. It is important to know how well an air curtain performs in a given operating scenario in order to assess its energy saving potential. In recent literature, it was proposed to use the waterbath experimental technique as a quick and inexpensive tool for testing the performance of an air curtain in a given scenario. The first central objective of the present study is to validate whether results obtained from small-scale waterbath experiments (WB) are adequate to describe the performance of a real-scale air curtain. To this end, we selected scenarios previously investigated by means of WB experiments and conducted real-scale measurements (RS) with a commercially available air curtain device (ACD) in a climate chamber, and RANS numerical simulations (NS) reproducing the configuration of the ACD in the climate chamber. These scenarios include (I) the base case of a downwards discharging air curtain when the doorway is the only connection between the two rooms, (II) the case when there is an additional top-level ventilation pathway between the rooms as well as (III) the case of a downwards blowing heated air curtain when the rooms are again joined only by the doorway. The second main goal of the present study is to verify the previously developed theoretical models for scenarios (II) and (III) by real-scale measurements and numerical simulations. As the main results, we establish that the effectiveness curves for an ACD obtained from WB experiments on the one hand as well as RS and NS measurements on the other exhibit the same general trends but also possess some minor differences for which we offer possible explanations. Furthermore, we find that RS and NS data are very well described by the previously proposed theoretical models in scenarios (II) and (III). The main finding of the present study is that the waterbath technique can be used as a reliable investigation tool to assess the general trends of the performance of an ACD in a given scenario but should be complemented by a small amount of RS or NS measurements if the finer details are important.