טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYael Shohat
SubjectModified Wind Chill Temperature based on Estimated Human
Convective Heat Transfer Coefficients and a Whole
Body Thermoregulation Model
DepartmentDepartment of Biomedical Engineering
Supervisors Professor Emeritus Shitzer Avraham
Professor Emeritus Degani David
Full Thesis textFull thesis text - English Version


Abstract

Convective heat loss to the surrounding is greater as wind speed increases. Thus, a person exposed to windy conditions looses heat faster than a person exposed to calm wind conditions at the same air temperature. The wind chill equivalent temperature (WCET) provides a measure of the equivalent calm air temperature that would result in the same cooling effect as under the actual environmental conditions. The current WCET is based on a steady state heat exchange model of the face, which incorporates a convective heat transfer coefficient correlation that is based on experiments with inanimate objects.

The first aim of this study was to estimate convective heat transfer coefficients between the human head and its surroundings that are based on human experimental data in cold and windy conditions. The second aim was to implement the estimated coefficients in a whole body thermoregulatory model to calculate WCETs.

The estimation of the convective heat transfer coefficients was done by numerically modeling the thermal interaction between the human head and its surroundings in cold and windy conditions. The model was solved using a commercial software package (Matlab). The convective coefficients were estimated by adjusting their values to obtain a satisfactory fit between the numerical predictions and the experimental data. Due to limitations imposed on human experiments under harsh environmental conditions, only limited sets of experimental data were available. The only environmental temperature, for which detailed human data were available over a range of wind speeds of 0.2 to 6 m s-1, was -10 ºC. The estimated coefficients were curve-fitted into a correlation depending on wind speed, environmental temperature and thermo-physical properties of air. The fitted correlation was extrapolated to higher wind speeds and other environmental temperatures due to the lack of experimental data. Both the estimated convective coefficients and the calculated WCETs were generally higher than those currently in use by the weather services in North America but followed a similar trend. A new correlation for the convective heat transfer coefficients of the face in cold and windy conditions is presented. This facilitates the estimation of more accurate skin temperatures than those obtained by the currently used correlation. Calculations of the WCETs using a whole body model, and not just a facial model, may yield more realistic predictions and thus better alert people for conditions of cold exposure. Further experimental data are required to generate more detailed and accurate convective heat transfer coefficients.