SESSION 4
BLOOD PERFUSION AND ENVIRONMENTAL EFFECTS
Chairman: B. Erdmann
THE THERMAL EFFICIENCY OF COLD-STRESSED FINGERS
Avraham Shitzer
Department of Mechanical Engineering,
Technion, Israel Institute of Technology,
Haifa, Israel 32000a
ABSTRACT
Thermal efficiency of cold-stressed finger-tips during cold induced vasodilatation (CIVD) is
considered. The actual heat loss from the finger-tip is compared to either the minimal or the
maximal heat losses. The actual heat loss is estimated by integrating the area under the time-
temperature curve of the finger-tip. The minimal heat loss is estimated by extrapolating an
exponential approximation of finger-tip temperature until it reaches a certain minimal value.
The value used in this study is 5oC, which is the pain threshold. The maximal heat loss is
calculated by assuming finger-tip temperature to be maintained at its initial value throughout
the cold exposure. These quantities were calculated for a series of exposures involving two
environmental conditions of gloved subjects: Tdry bulb= - 17.2oC, Tdew point= - 25.1oC (cold-dry)
and Tdry bulb= 0oC, Tdew point = - 8.4oC (cold-wet). Thermal efficiency was in the range of
0.40 - 0.85 for the minimal heat loss value (hmin) and 0.22 - 0.72 for the maximal heat loss
value (hmax). Weak linear relationships between the two definitions of the thermal efficiencies
and the total duration of the CIVD phase was indicated. The thermal efficiency based on
minimal heat loss indicated an inverse relation with the total duration of the CIVD phase. This
contradiction could be reconciled by the application of the common concept of "coefficient of
performance". Considerable inter- and intra-subjects variability was found.
a This work was done while the author was a National Research Council Senior Research Associate
funded by the Defense Health Research Grant No. p633002.801. It was also supported by the Fund for
the Promotion of Research at the Technion.
MEASUREMENT OF THERMAL DIFFUSIVITY OF BOVINE AORTA SUBJECT TO FINITE DEFORMATION
David J. Doss, Jay D. Humphrey1, and Neil T. Wright
Department of Mechanical Engineering, University of Maryland, Baltimore County
Baltimore, Maryland 21250 USA
ABSTRACT. The flash thermal diffusivity measurement technique is applied to tissue for
the first time. Making use of its minimal contact with the specimen, the flash technique is
extended to allow for well-defined, biaxial, finite strain. As an example application, the
radial component of thermal diffusivity of bovine descending aorta is measured in vitro as
a function of equibiaxial stretch, at room temperature. Data analysis is accomplished
using a Marquardt algorithm coupled with a finite difference solution of the thermal
diffusion equation. Extension of this method to measure simultaneously three orthogonal
components of diffusivity, at different levels of temperature, is discussed.
1 Current Address: Biomedical Engineering Program, Texas A&M University, College
Station, Texas, 77843-3120, USA
THE EFFECT OF TURBULENT FLUCTUATION OF AIR ON THERMAL SENSATION
Kiyomi Sugaia, Hiroshi Maekawab, Mutsuo Kobayashib, Tsuyoshi Takanob and Satoshi Komoriyab
a Department of Human Life and Environmental Science, Niigata Women's College, Niigata 950-8680, Japan
b Faculty of Engineering, Niigata University, Niigata 950-2181, Japan
INTRODUCTION
At present, many excellent air conditioning systems which are people-sensitive or
are equipped with fuzzy controls have been developed. However a quantitative
method for assessing their comfort or discomfort has not yet been sufficiently
developed. In the PMV index [1], for example, the effects of air flow on human
thermal comfort were investigated thoroughly. Recently, using not only steady flow is
but also unsteady flows with their velocity varying periodically, the effects of airflows
have been discussed. F.H.Rohles et al. [2] investigated the effects of periodic air now
on thermal sensations under the condition of mean velocities of 0.2, 0.4 and 0.8 m/s.
Tanabe et al. [3] examined the effects of slowly oscillating air flows on thermal comfort
in air-conditioned spaces. Though the air flow is in the usual air-conditioned space and
natural wind are mostly turbulent, there is no study which assesses quantitatively the
effect of turbulent velocity fluctuation on thermal sensation.
The aim of this study is to quantify how much influence the turbulent velocity
fluctuation of air flow exerts on thermal sensation, and further to investigate the reason
why the turbulent velocity fluctuation affects the thermal sensation from the point of
view of heat transfer.
EXPERIMENTAL METHODS
Two kinds of experiments were conducted: one measured thermal sensation and
skin temperature, and the other measured heat transfer from an aluminum disk which
simulated the palm of a hand.
The experiment was carried out in August. Twenty healthy, 21-to-25-year-o1d
males served as subjects. The subjects were seated at the side of the wind tunnel and
exposed the palm of their right hands to the turbulent and laminar flows of air
alternately. Velocities of air flows were kept at the same level of 0.8 m/s. The relative
intensity and the transverse integral scale of turbulence in the turbulent flow were 11.3%
and 6.1 mm. The temperature of the turbulent flow was kept constant at 20oC, while
that of the laminar flow was changed at intervals of 0.4oC between 17.2oC and 21.6oC.
After 60 seconds of exposure, that is, two times 15 seconds for each air flow, they stated
which air flow was felt warmer. The temperature of the laminar flow was changed in
successive 90 seconds, then the process was repeated. Each trial of the experiment
lasted 56 minutes.
To estimate the heat transfer coefficient from the palm, another experiment was
conducted using an aluminum disk as a model of the palm. After being preliminary
cooled, the disk was exposed to the same flows of air at a constant temperature as were
used in the subject experiments. The heat transfer coefficients were calculated from
the temporal change of the disk temperature and the fixed air temperature.
RESULTS
The thermal sensations for the two air flows when the temperature of the laminar
flow was decreased are shown in Fig.1. The abscissa indicates the temperature
difference when the temperature of the turbulent flow was subtracted from that of the
laminar flow. The ordinate indicates the rate of subjects who felt warmer for each air
flow. A point of intersection of two curves in Fig.1 became -0.6o, that is, the thermal
sensations for the two air flows were felt to be the same when the temperature of the
turbulent flow was 0.6oC higher than that of the laminar flow. This also signified that
the temperature was felt to be 0.6oC lower when the turbulent velocity fluctuation was
added to the air flow, even when the air temperature was the same. When the
temperature of the laminar flow was increased, the point of intersection became -1.4oC.
The difference between the temperature of the intersections obtained in the
temperature-increasing and decreasing series is ascribable to the psychological
expectation that the temperature of the laminar flow will soon become the same as that
of the turbulent flow. Accordingly it was quantified that the air temperature came to
be felt about 1.0oC lower by adding turbulent velocity fluctuation, if the abscissae of the
points of intersection were simply averaged to remove the influence of the direction of
temperature change. As this result was obtained under specific experimental
conditions, the temperature difference of the air flows obtained cannot be generalized.
flowever, it was found in this study that the turbulent flow is always felt as cooler than the
laminar flow with the same temperature and velocity.
The data obtained in the heat transfer experiment using the model of the palm were
substituted into an equation for the energy balance of the disk to calculate the heat
transfer coefficients. The coefficients from the disk to the turbulent and laminar flows
were 23.1 and 18.8 W/m2 oC. Similar to the experimental results of Kestin et al. [4] on
heat transfer from a cylinder, the heat transfer coefficient increased more in the
turbulent flow than in the laminar flow.
DISCUSSION
The heat transfer experiment using the model of the palm revealed that the heat
transfer coefficient from the palm in the turbulent flow was larger than that in the
laminar flow. Therefore it was expected that the temperature of the turbulent flow
would be felt lower because the heat transfer coefficient from the skin surface to the
environment increased in comparison with the laminar flow. This leads to the
hypothesis that the two air flows with the same velocity are felt to be at the same
temperature if heat losses from the skin to the environment are equal. Heat losses for
the turbulent and laminar flows of air, QL and QT, are given as follows:
 ...(1)
 ...(2)
where hL and hT are the heat transfer coefficients (W/m2 oC) in the turbulent and laminar
flows, A the surface area of the palm (m2), qs the skin temperature (oC), and qaL
and qaT the temperatures of the turbulent and laminar flows, respectively. As the two air
flows are felt to be at the same temperature when QT = QL according to the above
hypothesis, the temperature difference is calculated from Eq. (1) and (2) so that
 ...(3)
By substituting the air and skin temperatures and the heat transfer coefficients into
Eq.(3), the temperature difference at which the two air flows were felt as if they were at
the same temperature was calculated as about 2.5o. There is a several times disparity
when comparison is made with the temperature difference of about 1.0oC obtained in
the thermal sensation assessment. Though there is no doubt that subjects obtained
their thermal sensation for air flow mostly from the change of heat loss from the palm,
it was deduced that heat losses from the back of hand and aperture of fingers were also
partly related to this sensation. Nevertheless the above temperature difference was
calculated using the heat transfer coefficient at the upstream stagnation point. It is
possible that the simple model used in the heat transfer measurement would induce the
disparity between the temperature difference based on the hypothesis and that obtained
in the thermal sensation assessment.
CONCLUSION
The results are summarized as follows:
- From the thermal sensation assessment it could be originally shown that the
turbulent velocity fluctuation had the effect of causing the air temperature to be felt
as lower than that of the laminar flow.
- It was found from the heat transfer measurement that the heat transfer coefficient
from the skin surface became larger in the turbulent flow than in the laminar flow.
This suggests the cooler sensation felt in the turbulent flow is caused by the increase
of the heat transfer coefficient.
- It was confirmed that the hypothesis that the turbulent and laminar flows of air with
the same velocity would be felt as if they were at the same temperature when heat
losses from the skin to the two air flows were equal, was not in conflict with the results
of these experiments.
REFERENCES
- Fanger, P.O. et al.1972. Thermal Comfort. McGraw-Hill. New York
- Rohles, F.H., J.E.Wood, & R.G.Nevins. 1974. ASHRAE Transactions. 80: 101-119.
- Tanabe, S., K.Kimura, T.Hara, & T.Akimoto. 1987. Journal of Architecture, Planning and Environmental Engineering. 382: 20-28.
- Kestin, J., P.F.Meader & H.H.Sogin. 1961. ZAMP. 12: 115-132.
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