Effect of hydraulic diameter of flow straighteners on turbulence intensity in square wind tunnel

Author: Seo Youngjin  

Publisher: Taylor & Francis Ltd

ISSN: 1078-9669

Source: HVAC&R Research, Vol.19, Iss.2, 2013-02, pp. : 141-147

Disclaimer: Any content in publications that violate the sovereignty, the constitution or regulations of the PRC is not accepted or approved by CNPIEC.

Previous Menu Next

Abstract

This article presents an evaluation of the effect of cell sizes on the reduction of the turbulence intensity in a turbulent flow through straight ducts. It was generally observed that the turbulence intensity was reduced as the hydraulic diameter of the flow straighteners was decreased. A numerical approach to approximate the flow straightener geometry with a square profile was validated from experimental studies, using a bundle of straws as a flow straightener. Experiments were conducted to evaluate the effect of a set of flow straighteners in a square duct on velocity and turbulence intensity. The flow velocity and turbulence intensity were measured using a hot wire anemometer at 16 transverse points in a square tunnel for airflow rates ranging from 10 to 76 m3/min (353 to 2684 ft3/min); the corresponding Reynolds numbers range from 57,000 to 110,000. Measurements were taken at three hydraulic duct diameters downstream of the wind-tunnel entrance. The measurement points were chosen to be the center of each of the 16 square areas that divided the tunnel cross section. There was no difference in the coefficient of variation of velocity, irrespective of the use or nonuse of straighteners. However, the turbulence intensity was reduced from 10.3% to 2.56% by the use of straws as flow straighteners when the flow speed was 1.4 m/s (4.6 ft/s). Based on this experimental validation, a numerical method was adopted to further evaluate the effect of straightener cell sizes ranging from 0.003 to 0.153 m (0.01 to 0.5 ft) on turbulence intensity reduction. Whereas a cell size of 0.153 m (0.5 ft) provided no effect, a 0.003 m (0.01 ft) cell provided the highest level of reduction possible based on the ability of the straighteners to remove swirl from the flow. The method developed here provides a means for estimating the effect of screens of different porosity and mesh size on turbulence reduction. This is especially helpful in designing HVAC systems, in which square ducts are common and turbulence reduction is useful for achieving a uniform airflow distribution.