Convective and radiative heat transfer coefficients for individual human body segments

 Human thermal physiological and comfort models will soon be able to simulate both transient and spatial inhomogeneities in the thermal environment. With this increasing detail comes the need for anatomically specific convective and radiative heat transfer coefficients for the human body. The present study used an articulated thermal manikin with 16 body segments (head, chest, back, upper arms, forearms, hands, pelvis, upper legs, lower legs, feet) to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients. The tests were conducted across a range of wind speeds from still air to 5.0 m/s, representing atmospheric conditions typical of both indoors and outdoors. Both standing and seated postures were investigated, as were eight different wind azimuth angles. The radiative heat transfer coefficient measured for the whole-body was 4.5 W/m² per K for both the seated and standing cases, closely matching the generally accepted whole-body value of 4.7 W/m² per K. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.4 and 3.3 W/m² per K when standing and seated respectively. In the forced convective regime, heat transfer coefficients were higher for hands, feet and peripheral limbs compared to the central torso region. Wind direction had little effect on convective heat transfers from individual body segments. A general-purpose forced convection equation suitable for application to both seated and standing postures indoors was h _c=10.3v ^0.6 for the whole-body. Similar equations were generated for individual body segments in both seated and standing postures. Received: 21 May 1996/Accepted: 27 November 1996     

Convective heat transfer characteristics of toothed leaves

Summary Model leaves were used to test the hypothesis that serrate leaves have more convective heat loss than entire leaves of the same average size. Convection coefficients were positively correlated with size of teeth, supporting the hypothesis. Experimental results were in close agreement with theoretical predictions, which assumed an inverse correlation between depth of serration and effective leaf dimension.Abbreviations A total surface area of model (cm²) - D characteristic dimension of model (cm) - h convection coefficient (cal cm^-2 min^{-1 o}K^-1) - I radiant energy flux (cal cm^-2 min^-1) - Q radiant energy absorbed by leaf (cal min^-1) - q_c convective heat transfer (cal min^-1) - q_e evaporative heat loss (cal min^-1) - q_k conductive heat loss (cal min^-1) - q_r radiative heat loss (cal min^-1) - T_a air temperature (^oK) - T₁ model temperature (^oK) - V wind speed (m s^-1) - emissivity - Stefan-Boltzmann constant (8.130x10^-11 cal min^-1 cm^{-2 o}K^-4)     

Convective heat transfer measured directly with a heat flux sensor

A heat flux disk has been developed that directly measures the convective heat transfer in W/m2. When the sensor is calibrated on an aluminum cylinder, the calibration constant obtained is greatest in still air. As air movement increases, the calibration constant is reduced with increasing convective heat transfer coefficient, 0.5%.W-1.m2.K. The influence of wind on the calibration value is greatly reduced when the sensor is attached to a surface with lower thermal conductivity. The local convective heat transfer coefficient (hc) of the human body was measured. The leg acts in a manner similar to that of a cylinder, with the highest hc value at the front facing the wind and the lowest approximately 90 degrees from the wind, and in the wake a value is obtained that is close to the average hc value of the leg. When hc is measured at several angles and positions all over the body, the results indicate that the body acts approximately as a cylinder with a hc value related to the wind speed as hc = 8.6.v0.6 W.m-2.K-1, where v is velocity.     

Determination of heat transfer coefficients in plastic French straws plunged in liquid nitrogen

The knowledge of the thermodynamic process during the cooling of reproductive biological systems is important to assess and optimize the cryopreservation procedures. The time–temperature curve of a sample immersed in liquid nitrogen enables the calculation of cooling rates and helps to determine whether it is vitrified or undergoes phase change transition. When dealing with cryogenic liquids, the temperature difference between the solid and the sample is high enough to cause boiling of the liquid, and the sample can undergo different regimes such as film and/or nucleate pool boiling.In the present work, the surface heat transfer coefficients (h) for plastic French straws plunged in liquid nitrogen were determined using the measurement of time–temperature curves. When straws filled with ice were used the cooling curve showed an abrupt slope change which was attributed to the transition of film into nucleate pool boiling regime. The h value that fitted each stage of the cooling process was calculated using a numerical finite element program that solves the heat transfer partial differential equation under transient conditions. In the cooling process corresponding to film boiling regime, the h that best fitted experimental results was h = 148.12 ± 5.4 W/m² K and for nucleate-boiling h = 1355 ± 51 W/m² K. These values were further validated by predicting the time–temperature curve for French straws filled with a biological fluid system (bovine semen-extender) which undergoes freezing. Good agreement was obtained between the experimental and predicted temperature profiles, further confirming the accuracy of the h values previously determined for the ice-filled straw. These coefficients were corroborated using literature correlations.The determination of the boiling regimes that govern the cooling process when plunging straws in liquid nitrogen constitutes an important issue when trying to optimize cryopreservation procedures. Furthermore, this information can lead to improvements in the design of cooling devices in the cryobiology field.     

Measurement of heat transfer coefficients in stirred single‐use bioreactors by the decay of hydrogen peroxide

Single‐use bioreactors are barely described by means of their heat transfer characteristics, although some of their properties might affect this process. Steady‐state methods that use external heat sources enable precise investigations. One option, commonly present in stirred, stainless steel tanks, is to use adjustable electrical heaters. An alternative are exothermic chemical reactions that offer a higher flexibility and scalability. Here, the catalytic decay of hydrogen peroxide was considered a possible reaction, because of the high reaction enthalpy of –98.2 kJ/mole and its uncritical reaction products. To establish the reaction, a proper catalyst needed to be determined upfront. Three candidates were screened: catalase, iron(III)‐nitrate and manganese(IV)‐oxide. Whilst catalase showed strong inactivation kinetic and general instability and iron(III)‐nitrate solution has a pH of 2, it was decided to use manganese(IV)‐oxide for the bioreactor studies. First, a comparison between electrical and chemical power input in a benchtop glass bioreactor of 3.5 L showed good agreement. Afterwards the method was transferred to a 50 L stirred single‐use bioreactor. The deviation in the final results was acceptable. The heat transfer coefficient for the electrical method was 242 W/m²/K, while the value achieved with the chemical differed by less than 5%. Finally, experiments were carried out in a 200 L single‐use bioreactor proving the applicability of the chemical power input at technical relevant scales.     

Liquid-phase mass transfer coefficients in bioreactors

Liquid-phase mass transfer coefficient in bioreactors have been examined. A theoretical model based on the surface renewal concept has been devloped. The predicted liquid-phase mass transfer coefficients are compared with the experimental data for a mycelial fermentation broth (Chaetomium cellulolyticum) and model media (carboxymethyl cellulose) in a bench-scale bubble column reactor. The liquid-phase mass transfer coefficient is evaluated by dividing the volumetric mass transfer coefficient obtained experimentally by the specific surface area estimated using the available correlations. The available literature data in bubble column and stirred tank bioreactors is also used to test the validity of the proposed model. A reasonable agreement between the model and the experimental data is found.     

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/m²°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/m²°C, respectively for 20, 30 and 45 steps/min.     

Adenoviral gene transfer to the neonatal rat pulmonary circulation

BACKGROUND: Gene delivery to the pulmonary circulation has been studied in adult animals, but has not been extensively investigated in neonates. METHODS: We tested the ability of recombinant, replication-defective adenovirus to transduce the pulmonary circulation when delivered by percutaneous ventricular puncture. Five-day-old rat pups were injected with 10(7) to 10(10) particles (approximately 10(5) to 10(8) pfu) in 30 micro l total volume. RESULTS: Using RT-PCR, we detected transgene expression in both lung and liver at all dosages. However, whereas only 1/6 pups injected with 10(7) particles had detectable expression, 8/9 pups in the two highest dose groups had detectable expression. In the highest dose group expression was approximately 5-fold greater in lung than liver, though in the lower dose groups no difference between lung and liver was found. Expression decreased by only 25% from day 4 through the last time point at day 28 in lung, whereas liver expression was undetectable in 7 of 9 samples on day 28. Histopathological examination demonstrated expression both within the media of large arteries and in small, peripheral arteries and capillaries, with a concentration of expression in the most distal areas of both the lungs and liver. No evidence of inflammation was seen. CONCLUSIONS: We conclude that the neonatal pulmonary circulation can be effectively transduced using systemic adenoviral vector injection, has more sustained expression than liver, and may be a target for therapeutic gene delivery.     

Oxygen transfer coefficients by dynamic model moment analysis

A new method to estimate the oxygen transfer coefficient (K_La) from the experimental dynamic response data is presented. Employing a linear model which allows for gas phase, diffusion film, and oxygen electrode dynamics, the first moment of the response curve is simply related to the sum of the model parameters. Two separate experiments are used to characterize the measurement dynamics and to measure the unknown K_La parameter. The simple calculation procedure involves only measuring the area above the response curves.     

Convective mass transfer effects on the intracellular calcium response of endothelial cells.

Endothelial cells, which line the vasculature, respond to specific agonists such as adenosine triphosphate (ATP) by elevating cytosolic calcium levels and increasing production of the vasoactive compounds, prostacyclin and endothelial derived relaxing factor (EDRF). Endothelial cells express ecto-enzymes which metabolize ATP. If the activity of these enzymes is sufficiently high, then the concentration of ATP near the endothelial cell surface can be substantially lower than the bulk concentration. The ATP concentration is determined by a balance between the convection of fresh ATP from upstream and the degradation of ATP by the endothelial cells. In this report, we present a parallel plate flow system for measurement of cytosolic calcium levels ([Ca2+]i) of individual bovine aortic endothelial cells with the calcium sensitive fluorescent dye, fura-2. The cells respond to increases in the flow rate by increasing [Ca2+]i if there is ATP present in the perfusing buffer, but not in the absence of ATP. The amount of agonist in the perfusing fluid near the endothelial cell surface is estimated by solving the governing differential equation for the concentration profile of ATP in the parallel plate flow geometry. The solution indicates that one mechanism endothelial cells may use to detect changes in the flow rate is to respond to the change in the local concentration of agonist.     

A metabolic derivation of tritium transfer coefficients in animal products

Tritium is a potentially important environmental contaminant originating from the nuclear industry, and its behaviour in the environment is controlled by that of hydrogen. Animal food products represent a potentially important source of tritium in the human diet and a number of transfer coefficient values for tritium transfer to a limited number of animal products are available. In this paper we present an approach for the derivation of tritium transfer coefficients which is based on the metabolism of hydrogen in animals. The derived transfer coefficients separately account for transfer to and from free (i.e. water) and organically bound tritium. A novel aspect of the approach is that tritium transfer can be predicted for any animal product for which the required metabolic input parameters are available. The predicted transfer coefficients are compared to available independent data. Agreement is good (R ²=0.97) with the exception of the transfer coefficient for transfer from tritiated water to organically bound tritium in ruminants. This may be attributable to the particular characteristics of ruminant digestion. We show that tritium transfer coefficients will vary in response to the metabolic status of an animal (e.g. stage of lactation, diet digestibility etc.) and that the use of a single transfer coefficient from diet to animal product is inappropriate. It is possible to derive concentration ratio values from the estimated transfer coefficients which relate the concentration of tritiated water and organically bound tritium in an animal product to their respective concentrations in the animals diet. These concentration ratios are shown to be less subject to metabolic variation and may be more useful radioecological parameters than transfer coefficients. For tritiated water the concentration ratio shows little variation between animal products ranging from 0.59 to 0.82. In the case of organically bound tritium the concentration ratios vary between animal products from 0.15 (goat milk) to 0.67 (eggs). Received: 28 May 2001 / Accepted: 20 August 2001     

Modeling of heat transfer in whole-body hyperthermia

The therapeutic application of whole-body hyperthermia, whereby the body temperature is for a short time raised to 43–44°C is currently considered quite promising. However, body temperature above 42°C also raises the risks associated with hemodynamic instability and arrhythmia. A model of heat transfer is built to improve the efficacy and safety of the immersion-convection technique of whole-body hyperthermia. The model takes into account the changes in skin blood flow and the dynamics of heart rate depending on body temperature. It adequately reflects the processes of heating in the organism and can be used to calculate the heat distribution in the body.     

Modelling heat transfer in a bone-cement-prosthesis system

The heat transfer in a general bone-cement-prosthesis system was modelled. A quantitative understanding of the heat transfer and the polymerization kinetics in the system is necessary because injury of the bone tissue and the mechanical properties of the cement have been suggested to be effected by the thermal and chemical history of the system. The mathematical model of the heat transfer was based on first principles from polymerization kinetics and heat transfer, rather than certain in vitro observed properties, which has been the common approach. Our model was valid for general three-dimensional geometries and an arbitrary bone cement consisting of an initiator and monomer. The model was simulated for a cross-section of a hip with a potential femoral stem prosthesis and for a cement similar to Palacos R. The simulations were conducted by using the finite element method. These simulations showed that this general model described an auto accelerating heat production and a residual monomer concentration, which are two phenomena suggested to cause bone tissue damage and effect the mechanical properties of the cement.