A comparison of CFD-simulations and measurements of the temperature stratification in a mixing box of an air-handling unit

Abstract

SUMMARYThis paper presents a comparison between CFD-simulations and measurements of the temperature strati"cation in a mixing box of an air-handling unit. We have used data from "eld measurements during a period of over a year for di!erent outside temperatures. We performed two-dimensional CFD-simulations for four di!erent outside temperatures with commercially available software. The measurements as well as the simulations show that the temperature di!erence between the upper part and the lower part of the duct downstream of the mixing box is considerable. It increases, as the outside temperature decreases. However, the discrepancies between the measurements and the simulations are large. The reasons for this are uncertain boundary conditions and modelling errors leading to an inaccurate simulation result. The strati"cation downstream of the mixing box implies large sensor errors and the use of the mixed air temperature for control and fault detection must therefore be questioned. Averaging sensors, which take a mean value over the duct section, can be used but do not consider di!erences in velocities and are therefore not accurate either. In order to, for example, use CFD as a tool to decide the optimal sensor location a more accurate model and more information regarding the boundary conditions is needed.

A fuzzy sensor for measuring the mixed air temperature in air-handling units

“…Previous work [16] has shown that the thermo-fluid behaviour inside a mixing-box can be simulated reasonably accurately using the CFD approach. Common features of the variations in the sensor bias errors with operating conditions are extracted from the simulation data using fuzzy identification techniques.…”