Fundamental studies on ventilation for improving thermal comfort conditions and indoor air quality

Abstract

A thermistor based air velocity sensor has been developed and tested that makes it possible to measure velocity (omni-directional) as well as turbulence in room air flow with thermal sensors. It has fast time response ($>$2 Hz) (much better than commercially available omni-directional sensors), and good directional response (much better than hot wires). Its self-heating errors are comparable to those of commercially available hot wire sensors. It is a rugged sensor, easy to build, repair, and operate.

Theoretical analysis from this research reveals that the shear stress on the ceiling, created by moving air jets, and the negative buoyancy force of the jet, determine the separation distance of non isothermal jets. An attempt was made to use shear stress as holding force to develop models for predicting the separation distances of cold ventilating jets from linear and square (radial) diffusers.

Analysis in this research showed that thermal buoyancy forces (rising thermal plumes from heating loads, falling negative buoyant air jets, etc.) play important, sometimes dominant, part in room air flow. A model based on effective total momentum (momentum from the initial jet velocity as well as from thermal buoyant forces) has been developed for predicting room average air velocities in various realistic ventilation conditions. It was aimed that the same model could be applied to essentially any type of diffusers. Comparisons of the predicted results with experimental results from 44 tests with 9 diffuser setups showed very good agreement.

A technique was proposed and validated in laboratory conditions that measures the outdoor air exchange rate to building occupants directly and accurately, using a few (e.g. 2 to 3) concentration measurements from the tracer processes. Relationship between the outdoor air change rate and the mean age of air was derived and results showed that measurements of mean age of air can be simplified with the proposed method.