Experimental determination of coefficient of heat exchanges by convection of human body

Convective heat transfer coefficient relating to evaluation of thermal environment of infant

Infants have a low capacity to thermally adapt to their environment and so sufficient consideration must be given to their thermal environment. In investigating an infant's thermal environment, the purpose of this study is to clarify the heat transfer coefficient in natural convection for the posture of an infant in a stroller. The heat transfer coefficients were measured by means of using a thermal manikin. The experimental thermal environment conditions were set for eight cases, at: 16 °C, 18 °C, 20 °C, 22 °C, 24 °C, 26 °C, 28 °C, and 30 °C, and the air and wall surface temperatures were equalized, creating a homogeneous thermal environment. The air velocity (less than 0.2 m/s) and relative humidity (50%RH) were the same for each case. The surface temperature of each part of the thermal manikin was controlled to 34 °C. The difference between the mean surface temperature and air temperature (ΔT [K]) is the driving force for the heat transfer coefficient in natural convection for the posture of an infant in a stroller (h_c [W/(m²·K)]). We propose the use of the empirical formula h_c = 2.16 ΔT ^{0 .23}. The formula of the convective heat transfer coefficient in natural convection of this study can be applied to infants up to about 3 years old.

A new causative heat supply for exertional heat stroke on runners in cold air

The dysregulation in heat balance, the main cause of exertional heat stroke, occurs not only in midsummer but also in the cold season. Possible causes of this are a reduction in convection and evaporation due to tailwinds and an acceleration of radiant heat inflow. Although the amount of radiant heat that reaches the surface can be estimated, the actual amount of heat that flows into the body cannot be specified yet. This paper made an experimental attempt at this. A device is made up of a temperature controllable heat sink and heat flow detector, which keeps the surface temperature constant and has a heat exchange coefficient comparable to that of the human body surface. The output of this device (total heat exchange) was divided into radiant heat exchange and other heat exchange using a standard radiant heat calibrator, Leslie cube. A phenomenon, in which a wet surface while the surface temperature was low absorbed larger heat than that of the dry surface, was found. And authors named this "hidden heat inflow". As a result of multiple regression analyses, both radiant heat exchange and other heat exchanges are closely related to the surface temperature, and the maximum difference in total heat exchange during the experiment reached 200 kcal/m²/h. It has been suggested that this phenomenon may also occur on the surface of human skin. One of the causes of this "hidden heat inflow" is considered to be the decrease in evaporative cooling due to the decrease in surface temperature. However, this alone cannot explain all of the phenomena, so water vapor aggregation may also be involved. A "hidden heat inflow" as a sufficient heat source for exertional heat stroke or collapse during a marathon race on a cold day was evidenced experimentally.

Bioheat Transfer Basis of Human Thermoregulation: Principles and Applications

Thermoregulation is a process that is essential to the maintenance of life for all warm-blooded mammalian and avian species. It sustains a constant core body temperature in the face of a wide array of environmental thermal conditions and intensity of physical activities that generate internal heat. A primary component of thermoregulatory function is the movement of heat between the body core and the surface via the circulation of blood. The peripheral vasculature acts as a forced convection heat exchanger between blood and local peripheral tissues throughout the body enabling heat to be convected to the skin surface where is may be transferred to and from the environment via conduction, convection, radiation, and/or evaporation of water as local conditions dictate. Humans have evolved a particular vascular structure in glabrous (hairless) skin that is especially well suited for heat exchange. These vessels are called arteriovenous anastomoses (AVAs) and can vasodilate to large diameters and accommodate high flow rates. We report herein a new technology based on a physiological principle that enables simple and safe access to the thermoregulatory control system to allow manipulation of thermoregulatory function. The technology operates by applying a small amount of heating local to control tissue on the body surface overlying the cerebral spine that upregulates AVA perfusion. Under this action, heat exchangers can be applied to glabrous skin, preferably on the palms and soles, to alter the temperature of elevated blood flow prior to its return to the core. Therapeutic and prophylactic applications are discussed.

Demonstration of treatment planning software for hyperthermic intraperitoneal chemotherapy in a rat model

BACKGROUND

Hyperthermic intraperitoneal chemotherapy (HIPEC) is administered to treat residual microscopic disease after cytoreductive surgery (CRS). During HIPEC, fluid (41-43 °C) is administered and drained through a limited number of catheters, risking thermal and drug heterogeneities within the abdominal cavity that might reduce effectiveness. Treatment planning software provides a unique tool for optimizing treatment delivery. This study aimed to investigate the influence of treatment-specific parameters on the thermal and drug homogeneity in the peritoneal cavity in a computed tomography based rat model.

METHOD

We developed computational fluid dynamics (CFD) software simulating the dynamic flow, temperature and drug distribution during oxaliplatin based HIPEC. The influence of location and number of catheters, flow alternations and flow rates on peritoneal temperature and drug distribution were determined. The software was validated using data from experimental rat HIPEC studies.

RESULTS

The predicted core temperature and systemic oxaliplatin concentration were comparable to the values found in literature. Adequate placement of catheters, additional inflow catheters and higher flow rates reduced intraperitoneal temperature spatial variation by -1.4 °C, -2.3 °C and -1.2 °C, respectively. Flow alternations resulted in higher temperatures (up to +1.5 °C) over the peritoneal surface. Higher flow rates also reduced the spatial variation of chemotherapy concentration over the peritoneal surface resulting in a more homogeneous effective treatment dose.

CONCLUSION

The presented treatment planning software provides unique insights in the dynamics during HIPEC, which enables optimization of treatment-specific parameters and provides an excellent basis for HIPEC treatment planning in human applications.

Thermoregulatory modeling for cold stress

Modeling for cold stress has generated a rich history of innovation, has exerted a catalytic influence on cold physiology research, and continues to impact human activity in cold environments. This overview begins with a brief summation of cold thermoregulatory model development followed by key principles that will continue to guide current and future model development. Different representations of the human body are discussed relative to the level of detail and prediction accuracy required. In addition to predictions of shivering and vasomotor responses to cold exposure, algorithms are presented for thermoregulatory mechanisms. Various avenues of heat exchange between the human body and a cold environment are reviewed. Applications of cold thermoregulatory modeling range from investigative interpretation of physiological observations to forecasting skin freezing times and hypothermia survival times. While these advances have been remarkable, the future of cold stress modeling is still faced with significant challenges that are summarized at the end of this overview.

Convective heat transfer from a nude body under calm conditions: assessment of the effects of walking with a thermal manikin

The present experimental work is dedicated to the analysis of the effect of walking on the thermal insulation of the air layer (I (a)) and on the convective heat transfer coefficients (h (conv)) of the human body. Beyond the standing static posture, three step rates were considered: 20, 30 and 45 steps/min. This corresponds to walking speeds of approximately 0.23, 0.34 and 0.51 m/s, respectively. The experiments took place in a climate chamber with an articulated thermal manikin with 16 independent parts. The indoor environment was controlled through the inner wall temperatures since the objective of the tests was restricted to the influence of the walking movements under calm conditions. Five set points were selected: 10, 15, 20, 25 and 30°C, and the operative temperature within the test chamber varied between 11.9 and 29.6°C. The highest and lowest I ( a ) values obtained were equal to 0.87 and 0.71 clo, respectively, and the reduction in insulation due to walking ranged between 9.8 and 11.5%. The convective coefficients (h (conv)) for the whole body and for the different body segments were also determined for each step rate. In the case of the whole body, for the standing static reference posture, the mean value of h (conv) was equal to 3.3 W/m(2)°C and a correlation [Nu = Nu(Gr)] for natural convection is also presented in good agreement with previous results. For the other postures, the values of h (conv) were equal to 3.7, 3.9 and 4.2 W/m(2)°C, respectively for 20, 30 and 45 steps/min.

Numerical studies on the microclimate around a sleeping person and the related thermal neutrality issues

This article reports on two numerical studies on the microclimate around, and the thermal neutrality of, a sleeping person in a space installed with a displacement ventilation system. The development of a sleeping computational thermal manikin (SCTM) placed in a space air-conditioned by a displacement ventilation system is first described. This is followed by reporting the results of the first numerical study on the microclimate around the SCTM, including air temperature and velocity distributions and the heat transfer characteristics. Then the outcomes of the other numerical study on the thermal neutrality of a sleeping person are presented, including the thermal neutrality for a naked sleeping person and the effects of the total insulation value of a bedding system on the thermal neutrality of a sleeping person. STATEMENT OF RELEVANCE: The thermal environment would greatly affect the sleep quality of human beings. Through developing a SCTM, the microclimate around a sleeping person has been numerically studied. The thermal neutral environment may then be predicted and contributions to improved sleep quality may be made.

A global bioheat model with self-tuning optimal regulation of body temperature using Hebbian feedback covariance learning

In the lower brain, body temperature is continually being regulated almost flawlessly despite huge fluctuations in ambient and physiological conditions that constantly threaten the well-being of the body. The underlying control problem defining thermal homeostasis is one of great enormity: Many systems and sub-systems are involved in temperature regulation and physiological processes are intrinsically complex and intertwined. Thus the defining control system has to take into account the complications of nonlinearities, system uncertainties, delayed feedback loops as well as internal and external disturbances. In this paper, we propose a self-tuning adaptive thermal controller based upon Hebbian feedback covariance learning where the system is to be regulated continually to best suit its environment. This hypothesis is supported in part by postulations of the presence of adaptive optimization behavior in biological systems of certain organisms which face limited resources vital for survival. We demonstrate the use of Hebbian feedback covariance learning as a possible self-adaptive controller in body temperature regulation. The model postulates an important role of Hebbian covariance adaptation as a means of reinforcement learning in the thermal controller. The passive system is based on a simplified 2-node core and shell representation of the body, where global responses are captured. Model predictions are consistent with observed thermoregulatory responses to conditions of exercise and rest, and heat and cold stress. An important implication of the model is that optimal physiological behaviors arising from self-tuning adaptive regulation in the thermal controller may be responsible for the departure from homeostasis in abnormal states, e.g., fever. This was previously unexplained using the conventional "set-point" control theory.

Convective heat transfer area of the human body

In order to clarify the heat transfer area involved in convective heat exchange for the human body, the total body surface area of six healthy subjects was measured, and the non-convective heat transfer area and floor and chair contact areas for the following nine common body positions were measured: standing, sitting on a chair, sitting in the seiza position, sitting cross-legged, sitting sideways, sitting with both knees erect, sitting with a leg out, and the lateral and supine positions. The main non-convective heat transfer areas were: the armpits (contact between the upper arm and trunk regions), contact between the two legs, contacts between the fingers and toes, and contact between the hands and the body surface. Also, when sitting on the floor with some degree of leg contact (sitting in the seiza position, cross-legged, or sideways), there was a large non-convective heat transfer area on the thighs and legs. Even when standing or sitting in a chair, about 6-8% of the body surface did not transfer heat by convection. The results showed that the effective thermal convective area factor for the naked whole body in the standing position was 0.942. While sitting in a chair this factor was 0.860, while sitting in a chair but excluding the chair contact area it was 0.918, when sitting in the seiza position 0.818, when sitting cross-legged 0.843, in the sideways sitting position 0.855, when sitting with both knees erect 0.887, in the leg-out sitting position 0.906, while in the lateral position it was 0.877 and the supine position 0.844. For all body positions, the effective thermal convective area factor was greater than the effective thermal radiation area factor, but smaller than the total body surface area.

A computer model of human thermoregulation for a wide range of environmental conditions: the passive system

A dynamic model predicting human thermal responses in cold, cool, neutral, warm, and hot environments is presented in a two-part study. This, the first paper, is concerned with aspects of the passive system: 1) modeling the human body, 2) modeling heat-transport mechanisms within the body and at its periphery, and 3) the numerical procedure. A paper in preparation will describe the active system and compare the model predictions with experimental data and the predictions by other models. Here, emphasis is given to a detailed modeling of the heat exchange with the environment: local variations of surface convection, directional radiation exchange, evaporation and moisture collection at the skin, and the nonuniformity of clothing ensembles. Other thermal effects are also modeled: the impact of activity level on work efficacy and the change of the effective radiant body area with posture. A stable and accurate hybrid numerical scheme was used to solve the set of differential equations. Predictions of the passive system model are compared with available analytic solutions for cylinders and spheres and show good agreement and stable numerical behavior even for large time steps.

Evaluation of hand and finger heat loss with a heated hand model

A heated full-scale hand model has been used to determine indirectly hand and finger heat losses of human subjects exposed to four ambient cold conditions (0, 4, 10 and 16 degrees C, air velocity approximately 0.3 m/s). Heat transfer coefficients determined with the hand model, were used to calculate heat flux based on measured skin to ambient temperature gradients. The responses of eight subjects from a previous study were used for the analysis. The measurements were carried out in a small climate chamber which was cooled by evaporating liquid carbon dioxide. The thermal hand was put into the chamber in a vertical position with the thumb up. The surface temperature of the thermal hand was controlled at 21, 25, 28, 31 and 34 degrees C under each of the four ambient cold conditions, in order to investigate possible temperature dependence of the calculated combined convective and radiate heat transfer coefficient (hCR). The value of hCR varied between approximately 9-13 W/m2 degree C for fingers and palm and back of hand, respectively. Calculated heat losses showed significant individual variation, corresponding to the maintained skin to ambient temperature gradient. Individual values from about 50 to more than 300 W/m2 were calculated. Several subjects showed CIVD and heat fluxes associated with this phenomenon were sometimes doubled. The measurement results showed realistic and comparable with literature date. The advantages of the thermal hand model can be counted as easy to use; directly measures the heat loss; highly reproducible and no interruption. It appears that a heated hand model provides a useful methods for analysis and quantification of hand heat loss.

Thermal balance in term and preterm newborn infants nursed in an incubator equipped with a radiant heat source

A radiant hood warmer, a device that heats the incubator roof independently of the incubator's main heat source, was used to study the thermal balance of 11 full term and 13 preterm (gestational age 25-34 weeks) infants exposed to an isolated elevation of incubator roof temperature at stable ambient air temperature and humidity. After initial measurements without active heating of the incubator roof, the hood warmer was set to 33 degrees C, 36 degrees C and finally (preterm infants only) to 39 degrees C. At least 18 min of measurements with the infant asleep were made at each hood warmer setting. In the term infants an increase in roof temperature from 30.5 degrees C to 35.6 degrees C resulted in an increase in skin temperature from 35.4 to 35.9 degrees C, and a decrease in radiative heat loss from 32.8 to 20.7 W/m2 exposed skin. In the preterm infants an increase in roof temperature from 31.0 to 38.4 degrees C led to an increase in skin temperature from 35.7 to 36.3 degrees C and a decrease in radiative heat loss from 34.1 to 13.0 W/m2 exposed skin. The increased inner roof surface temperature did not affect evaporative or convective heat loss, skin blood flow, respiratory water loss, oxygen consumption or transepidermal water loss in either group. Thus, at stable ambient air temperature and humidity, the increase in incubator roof temperature resulted in an increase in skin temperature and a decrease in radiative heat loss in both term and preterm infants.

A simplified procedure of direct calorimetry for bedside monitoring of the resting metabolic rate

A simplified procedure of direct calorimetry (SPDC) for determination of resting metabolic rate of respiratory uncompromised subjects in a supine position is presented. This procedure was based on computer-assisted measurements of heat losses due to evaporation, radiation, conduction, and convection. The subject's total loss of mass was recorded hydraulically with a beam scale and afterwards transformed into a digital electric signal. Differences between dry bulb temperature and mean skin temperature were measured by semiconductor thermistors and also transformed into digital signals. With special software an interfaced personal computer assisted in performing SPDC and in calculating heat losses due to evaporation, radiation, and conduction. In a thermoneutral environment, six healthy volunteers were investigated to determine the mean convective heat transfer coefficient (hc) from the difference in an individual between the metabolic energy transformation (M) measured by indirect calorimetry (IC) and the sum of heat losses by radiation, conduction, and evaporation. The room-specific value of hc of 2.12 (SD 0.22) W.m-2.degrees C-1 was in good agreement with data in the literature. Compared to the results of M from a second series of IC, the total heat loss (THL) measured by SPDC in a thermoneutral environment was calculated as 100.5 (SD 6.0)%. The THL by SPDC performed three times at 3-h intervals on ten other volunteers revealed a mean difference of 0.22 (SD 1.74) W.m-2. Thus, SPDC would seem to be a valid and reproducible method under conditions of thermal neutrality.

Consequences of partial body warming and cooling for the drives to local sweat rates

Climatic chamber experiments were carried out on young, healthy male students. The ambient temperature was 36 degrees C, while local warming of one extremity was compensated for by heatflow-equivalent cooling of the ipsilateral extremity by on-line calculation of the heat balance. When warming the arm and cooling the leg (type 1 experiments), a slight, but not statistically significant increase of local sweat rates at these extremities was recorded. However, when cooling the arm and warming the leg (type 2 experiments), both corresponding local sweat rates declined. The divergent results are interpreted in terms of previously reported different central weighting factors for skin temperatures as determined: (1) by the weighting for the area, or (2) by the weighting for the area and the sensitivity of the local sweat rate to warming and cooling. This means that the central processing of the mean skin temperature may be different for cooling and warming and that in both cases values can be different from recorded (area weighted) skin temperature. Calculating this modified mean skin temperature, we conclude that type 1 experiments may be interpreted by the hypothesis that the central regulator has a status very near an overall heat-balance, whereas type 2 experiments, although also carried out at heat-balance, may be centrally evaluated as predominant cooling. In these experiments again the central drives representing the whole body thermal state seem to override both the direct and centrally mediated local drives.

Transepidermal water loss in newborn infants. VI. Heat exchange with the environment in relation to gestational age

The amount of water evaporated from the skin of newborn infants and the temperatures of the skin, of the ambient air, and of the surfaces facing the infants were measured and used as a basis for calculation of the evaporative, radiative and convective heat exchange between the infant and the environment. The infants were of varying gestational ages, from 25 to 39 completed weeks of gestation. Evaporative heat exchange was high in the most preterm infants when nursed at a low ambient humidity, while the high ambient humidity needed to maintain these infants at a stable body temperature led to a low loss of heat through radiation and convection or even a heat gain. In the more mature infants evaporative heat exchange was lower, while radiative and convective heat exchange was higher.

Effect of heat shielding on convective and evaporative heat losses and on radiant heat transfer in the premature infant

Ten premature infants nursed on servocontrolled radiant warmer beds were studied in three environments designed to alter one or more factors affecting heat transfer (convection, evaporation, and radiation). In the control environment, infants were nursed supine on an open warmer bed. The second environment (walled chamber) was designed to reduce convection and evaporation by placing plastic walls circumferentially around the bed. In the third environment convection and evaporation were minimized by covering infants with a plastic blanket. Air turbulence, insensible water loss, and radiant warmer power were measured in each environment. There was a significant reduction in mean air velocity in the walled chamber and under the plastic blanket when compared to the control environment. A parallel decrease in insensible water loss occurred. In contrast, radiant power demand was the same for control and walled environments, but decreased significantly when infants were covered by the plastic blanket. This study suggests that convection is an important factor influencing evaporation in neonates nursed under radiant warmers. The thin plastic blanket was the most effective shield, significantly reducing radiant power demand.

Transepidermal water loss in newborn infants. V. Evaporation from the skin and heat exchange during the first hours of life

The amount of water evaporated from the skin was studied in 10 healthy newborn infants from their first minute of life, while being taken care of in the delivery room, and in 11 infants treated in incubators from their 30th min of life. The heat lost by evaporation, radiation and convection was calculated. Evaporation from the skin was very high during the first minutes after birth and was the main cause of heat loss during the first 15-30 min of life. Thereafter the amount of heat lost depended on the conditions under which the infant was nursed. Higher convective and radiative heat losses were found in delivery rooms than in incubators.

Thermometry of the surface of human skin. A study on a model using thermocouples, thermistors, thermovision and thermodyes

The temperature of the surface of a model of human skin is measured using a recently developed probe to house a thermocouple or thermistor. The results are compared with thermovision and thermodye measurements. The effect on skin temperature of hot and cold vessels at various depths is investigated.

[Experimental determination of coefficient of evaporative heat loss in still air (author's transl)]

The authors have determined the coefficient of evaporative heat loss of the human body (he) by means of humidity steps in low air movement (Va less than or equal to 0,2 m/s). Such a determination requires a fully wetted skin and this implies therefore some loss of dripping sweat. The collection of this dripping sweat allows the determination of the total evaporation: this evaporation exists on the skin surface and around the drops during their fall from the skin to the oil pan where dripping sweat is collected. An estimation of this dripping sweat evaporation allows to assess the skin evaporation and, consequently, the evaporative coefficient he. In these experimental conditions: E = S - SNE - 0,0005 SNE (PsH2O - PaH2O) where E is the skin evaporative rate (g/h);S = total sweat rate (g/h);SNE = the nonevaporative sweat rate (g/h);PaH2O = the partial pressure of saturated water (at Ts) on skin (mb) and PaH2O the partial pressure of water vapor in ambient air (mb). The coefficient of evaporative heat loss in low air movement thus found, is 5,18 +/- 0,22 W/m2-mb.