Heat and Moisture Transfer between Human Body and Environment

Jean-Paul Fohr

• Publisher: Wiley-ISTE
• Year: 2015
• ISBN:
  9781119245605
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Mechanics

Human adaptation under cold or hot temperatures has always required specific fabrics for clothing. Sports or protective garment companies propose to improve performance or safety. Behind thermal comfort lays many physical/physiological topics: human thermoregulation loop, natural or forced convection, heat and vapor transfer through porous textile layers, solar and infrared radiation effects. This book leads through progressive and pedagogic stages to discern the weight of all the concerned physical parameters.

1
Building a Model for a Coupled Problem

There are numerous and varied heat and humidity exchange coupled problems in the environment, and more specifically in man’s surrounding environment (comfort, habitat, clothing, etc.) and a common methodology to approach these can be established. First of all, we need to position ourselves in relation to the digital/IT tools currently offered in “the market” and which allow for a resolution of numerous physics problems. The readers may be under the impression that the difficulty resides rather in making a choice among all these tools/software. It is common for specialized research departments to use software adapted to their fields (habitat, aviation, automobile, etc.) though a layman perceives them as some sort of magic “black box”. When results come out, the reliability interval is often uncertain, as the given problem was never treated for a neighboring configuration. It should be noted that solving a mathematical model numerically with elaborated software presupposes the formulation of a number of simplifying hypotheses that may be valid for a given configuration, but risky for another. To take an extreme example, outside of our field of study, media report on the progress of IPCC works concerning climate heating predictions while they highlight the uncertainty of 20-year predictions. At planetary scale, ocean/atmosphere models are particularly complex.

Let us therefore consider a “system” whose thermal and hydric behavior in particular conditions is to be determined: an individual in a room, a manned vehicle, an incubator, a piece of sportswear, etc. It is always possible to set the proper orders of magnitude for the behavior of a system under thermal constraints by scale analysis of the equations of an adapted model and by using the theoretical and experimental data in the literature. This first model can be preliminary to the use of software that is more complex but more difficult to interpret under the relative influence of input parameters. Through several simple examples, we will examine the implementation of such models.

1.1. Basic equations of the models (Appendix 1)

A fluid medium (humid air, liquid water, etc.) put in motion by a machine (forced ventilation, pump, etc.), wind, temperature gradients (natural convection), can be described by a number of variables depending on space and time: pressure p, temperature T, velocity

, density ρ, enthalpy h, etc.

The (quite) general conservation equations given here are written in condensed notation, using a pseudo vector

(nabla), or gradient, which in Cartesian coordinates x, y, z can be written:

.

Mass conservation can be written as:

[1.1a]

Or if we use the differential operator in the direction of movement

, we then have:

[1.1b]

For vapor contained in incompressible air, mass conservation is written as:

[1.2]

This equation is based on Fick’s law of diffusion, which gives the mass diffusive flux

(kg/m²s) of the vapor species (ρ_v) in the air (ρ):

...

Table of contents

• Cover
• Table of Contents
• Title
• Copyright
• Preface
• 1: Building a Model for a Coupled Problem
• 2: Approximate Determination of Transfer Coefficients
• 3: Human Thermal Models
• 4: Heat and Humidity Transfer in Clothing
• APPENDICES
• Bibliography
• Index
• End User License Agreement