Numerical predictions and measurement of non-isothermal airflows : evaluation of buoyant wall jets and airflow through large openings

University dissertation from Linköping : Linköpings universitet

Abstract: How can we attain a good indoor climate and at the same time reduce energy consumption? These issues are of particular interest in countries with cold climates, where the need for heating is large. It is important to design a HV AC (Heating, Ventilation and Air Conditioning) system that achieves a desirable indoor climate, where discomfort due to draft and thermal discomfort is avoided. Therefore, one needs to optimize ventilation to get the best possible indoor climate for the lowest cost and at the same time avoid unnecessary heat losses, such as air leakage through openings and the building envelope. One method is to use numerical calculations such as Computational Fluid Dynamics (CFD) for analyzing the flow and thermal behavior, which can lead to better design of HVAC systems.The purpose of this study has been divided into two parts: to evaluate non-isothermal wall jets, and to compare airflows through large openings with and without air curtains. The goal of the first part was to evaluate the flow and thermal characteristics of buoyant wall air jets by using numerical predictions and full-scale measurements. The purpose of the second part was to numerically examine how the temperature difference, between the indoor and the outdoors, affects the airflows through large openings. These vertical openings were evaluated both with and without air curtains.In the evaluation of the warm plane air jet and the cold wall jet, full-scale measurements together with numerical predictions were used. The measurements were conducted in a specially designed test room where the streamwise development of the jet and the jet characteristic was captured. In the warm plane air jet case, the velocity was measured with a single component hot-wire anemometer, while for cold wall jet, the velocity and turbulence was captured with a two-component anemometer. For the numerical prediction of the warm plane air jet, three different k-B models were used and evaluated. The numerical calculation of the cold jet was made with a Reynolds Stress Model (RSM).The numerical predictions of the warm and cold wall jets showed good agreement with the experiments for the decay of mean velocity, mean velocity profiles, growth or spread of the jet, and decay of temperature. The predictions on the cold wall jet showed some deviations regarding the turbulent characteristics of the cold wall jet. To diminish any errors due to numerical prediction, a thorough procedure was employed in accordance with the applied turbulence model, near wall treatment, mesh density, solution accuracy in order to achieve the best possible result. In order to extend the ability of turbulence modeling the plan is to use Large Eddy Simulation (LES) in the future for further validation with available measurements.For further investigation of non-isothermal wall jets, it is recommended to use other ways of predicting and analyzing the turbulence in the flow. When applying a RSM, one should evaluate the dependence of the pressure-strain model. The use of unsteady calculations like URSM or LES is worth evaluating. It would also be desirable to use other measuring techniques such as LDA (Laser Doppler Anemometry) to capture the characteristics of, especially cold (negatively buoyant) wall jets.In the second part of this work, airflow through large vertical openings was examined. Here two-dimensional numerical predictions were performed with a k-ε model. The computations were compared with other numerical predictions and also with some  empirical formulas. First the amount of air that enters through a large opening was captured. Second the opening was supplied with an air curtain to evaluate its impact on the amount of incoming air.The use of an air curtain in the opening was shown to give a significant impact on the thermal indoor climate. When there was no protection in the opening the temperature ; in the building falls to nearly the outdoor temperature. But when an air curtain is installed, the cold outdoor air is prevented from entering the building, and thus the temperature in the building is maintained at a desirable level. Although an air curtain provides a good indoor temperature, it also produces significantly higher air velocities, which can cause discomfort due to draft near the opening. The results show that CFD calculations can be used for simulating the airflow in large openings with and without air curtains.For future work on the evaluation of airflows through large openings several topics  could be interesting to examine. Special focus should be given to the design of air curtains in the opening for preventing the cold air from entering the building. An interesting parameter to examine is the flow angle on the air curtain, to see if more heat from the curtain can be directed into the building instead of to the outside. The impact of a larger building also has to be evaluated, especially when an air curtain is used in the opening. Comparison with experiments would also be desirable for a more detailed calibration of the numerical results.

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