Nagham Ismail

Predicting segmental and overall ventilation of ensembles using an integrated bioheat and clothed cylinder ventilation models

The purpose of this work is to implement an integrated ventilation and heat transport model of a clothed human subject to external wind. Each segment of the human body is modeled as a vertical cylinder with an air annulus separating the ventilated clothing from the skin with open or closed end to the environment. The developed ventilation model takes into consideration natural convection induced by the temperature difference between the skin and the annulus air. The steady-state mass and energy balance equations of the annulus microclimate air and clothing were solved numerically. The model is integrated with a segmental bioheat model to determine the local skin temperature, which is used as boundary condition. The clothing ventilation model was validated by conducting experiments on an isothermally heated vertical clothed cylinder with an open lower end aperture and subject to a uniform cross wind. Good agreement was found between the model predictions and experimental measurements of temperature at different angular and vertical locations in the air layer. The combined clothed cylinder and segmental bioheat model was validated with published experimental data on ensemble total ventilation for different permeabilities and wind speeds. It was found that clothed segments opened from the bottom increased ventilation by 40% when compared to clothed segments opened from the top. An increase of wind speed by 1.0 m/s leads to an increase in ventilation rate of about 36%. Natural convection was also found to enhance the ventilation of highly permeable clothing at low wind speeds compared to less permeable clothing.

Effect of inter-segmental air exchanges on local and overall clothing ventilation

The aim of this work is to study the effect of the connection between arm and trunk segments in changing the flow characteristics and local ventilation. A model is developed that solves coupled momentum, mass and heat balances, including buoyancy for the connected clothed upper human body. The model was validated by performing computational fluid dynamics simulations to compare the microclimate air flow characteristics and flow direction at the connections. In addition, the model was also validated by comparing predicted overall ventilation with published data. The interconnection air exchanges affected significantly local ventilation in the trunk segment and the direction of the flow in the open-aperture-clothed arm segment. It was found that at relatively high wind speed (Vw ≥ 0.9 m/s) and with a permeable jacket, the inter-segmental ventilation became important and exceeded 5 l/min. Meanwhile, this inter-segmental ventilation caused an increase of 15% of trunk ventilation and a reduction of 4% of arm ventilation. The inter-segmental ventilation vanished and the air exchange between the trunk and the arm was no longer important at low permeability (α = 0.05 m/s) and at low wind speed (Vw = 0.1 m/s). Finally, the inter-segmental ventilation was more important for the open clothed arm aperture compared to when it was closed.

Determination of segmental and overall ventilation of clothed walking human by means of electric circuit analogy

A new simplified model has been developed to determine the ventilation induced by swinging motion and external wind for a fabric clothed cylinder representing a limb or a trunk. The simplified model is based on an analogy between air flow and an electric circuit. When a clothed body segment is subject to external wind, the microclimate air flow electric circuit is represented by resistances. When the clothed segment is subject to a swinging motion, the air flow electric circuit is composed of inductance and resistance elements. The model is validated by comparing the predicted ventilation rates to published experimental data in different conditions: varying permeability, wind speeds, swinging frequencies (for the clothed arm), walking conditions, and aperture configurations. The predictions of the simplified model lie within the standard deviation range of the published experiments. Moreover, although it is simplified, the relative error between the simplified model and the published experiment of an oscillating limb is considered acceptable (18%).

Improving local ventilation prediction by accounting for inter-segmental ventilation

The inter-segmental ventilation rate at clothing inter-connection of arms and trunk affects the estimation of local ventilation rates of these clothed segments. The accurate estimation of the inter-segmental ventilation rate is based on the integration of a connected clothed cylinders model with a bio-heat model to predict a realistic segmental skin temperature. This integration is validated with experiments on a thermal manikin using the tracer gas method. The results show that accounting for the inter-segmental ventilation rate improves the estimation of the segmental ventilation of the arm and the trunk for different garment apertures at external wind velocities less than 4 m/s. For a wind velocity of 1 m/s, the inter-connection increased the trunk ventilation by up to 12% and heat loss by up to 5.46%. A statistical correlation is established for the inter-segmental ventilation rate in terms of the influencing parameters: air permeability, wind velocity, mean air gap size between skin and clothing, and the upper clothing aperture design. Furthermore, a local ventilation rate correction factor equation is developed as a function of the inter-segmental ventilation rate to correct for local ventilation rates when derived from values of isolated/unconnected clothed segments.

Theoretical and Experimental Estimation of Inter-Segmental Clothing Ventilation and Impact on Human Segmental Heat Losses

Air exchange between a specific garment and the environment could occur 1) through the fabric with the environment, 2) through garment apertures with the environment, and 3) between local body parts’ microclimates. The first mechanism is related to the fabric properties and the flow characteristics around the human body. The second mechanism is induced by buoyancy and pressure alteration due to external wind. The third mechanism named inter-segmental ventilation occurs between different clothing sections caused by position of apertures, relative wind, fabric permeability and microclimate size of connected clothed segments. The objective of this work is to develop a simplified accurate model that solves coupled momentum, mass and heat balances including buoyancy for the connected clothed upper human body to predict inter-segmental ventilation and assess its impact on the air flow characteristics in the microclimate layer and on local ventilation rates. This model is coupled to the bioheat model to predict the effect of the inter-segmental ventilation on the heat losses from the body and on bringing the thermal comfort. The model is validated by performing an improved experimental method on a thermal manikin using the tracer gas method at different wind speeds for permeable clothing.Copyright

A Clothing Ventilation and Heat Loss Electric Circuit Model with Natural Convection for a Clothed Swinging Arm of a Walking Human

ABSTRACT This work aims to develop a computationally effective electric circuit model to estimate the ventilation and heat transfer for walking human in the presence of natural convection. The ventilation circuit includes flow resistance, inductance, and electromotive force elements. It is coupled to an electric resistance circuit of heat flows to adjust the temperature difference inducing natural convection flow. The coupled ventilation and heat circuit models predicted both the segmental ventilation rate and heat loss from the arm at different walking and wind speeds. The developed model of the segmental ventilation and heat transfer from the clothed human segment was validated by performing experiments on a walking thermal manikin using tracer gas method. Good agreement was observed between the model predictions and the experiment at a maximum relative error of 10% lying within the standard deviation range. Results showed that the simplified ventilation-heat circuit models succeeded in estimating the natural convection effect at low computational cost. Moreover, it was shown that the effect of natural convection is more significant in walking at no wind than in windy condition. Accounting for natural convection effect increases the segmental ventilation and heat loss at low air permeability (0.02 m/s) by 68% and 20%, respectively.