SKIN temperatures (57 C)the iatent neat or vap moi water ny of watch p=998.2 kg/m'. 3. Assume that the thickness of the tissue between the interior and the exterior of the body is d =3 cm and that the average area through which conduction can occur is A =1.5 m². Find heat transferred by conduction per hour if temperature difference between the inner body and the skin is AT = 2°C. Coefficient of thermal for tissue without blood is K. = 18 × 10-2 Cal /m-hr-ºC. 06 ".ll 59I 8:13 PM 4G Vodafone UA Heat_transfer_mec... THERMODYNAMICS Heat transfer mechanisms A Theory We know from experience that when two bodies, one hot and the other cold, are placed in an enclosure, the hotter body will cool and the colder body will heat until the degree of hotness of the two bodies is the same. Clearly something has been transferred from one body to the other to equalize their hotness. That which has been transferred from the hot body to the cold body is called heat. Units of heat are Joules(J) and calories (cal). Relationship between units 1 cal = 4.18 J and 1 Cal = 1000 cal = 4180 J 1. The quantitative relationship between heat transfer Q and temperature change AT contains three factors: Q = mcAT = mc(T, – T2), where Q is the symbol for heat transfer, m is the mass of the substance in kg (m = pV), and AT is the change in temperature. The symbol c stands for specific heat and depends on the material (J/kg-°C). The specific heat is the amount of heat necessary to change the temperature of 1.00 kg of mass by 1.00°C . 2. Energy eliminated during a phase change (vaporization/condensation) is determined by equation: Q = mLp, where Q is the symbol for heat transfer, m is the mass of the substance in kg pV), latent heat of vaporization Ly (J/kg). (m = Solid Metal rod Air convection Radiation Heat flow c Radiation 3. The amount of heat transferred by conduction Qc is given by b Convection a Conduction KçA(T1-T2) Qc = t = hconductionA ATt, here A is the area of the contact (m²), L it is length (m), and T – T2 is the temperature difference between the two ends (°C ). The constant K. is the coefficient of thermal conductivity (J/m-sec.°C), t is time (sec), hconduction= coefficient of conduction heat transfer (J/m2-sec.°C), d is the thickness of the material (m). Conduction Through Multiple Layers: when more layers are added, we would expect the rate of Kc heat transfer to decrease: 1 +... hrotal hiayeri hiayer2 4. The amount of heat transferred by convection Q. is given by Q. = K{A.(T, – Ta)t, here Ac is the area exposed to convective currents (m²); T, and Ta are the skin and air temperatures, respectively (°C); K/ = 8.3v0.6 is the coefficient of convection, which is a function of the velocity v of the convective fluid (J/m²-sec.°C), t is time (sec). For air K = 8.3v0.6 , here v is air velocity. 5. The amount of heat transferred by radiation Q, (J) is given by Qr = k,eA,(T – T,^)t where T, and T, are the skin surface temperature and the temperature of the nearby radiating surface (K), respectively; A, is the area of the body participating in the radiation (m²); e is the emissivity of the surface; 1 and Kr is the radiation coefficient. Over a fairly wide range of temperatures, k, is, on the average, about k, = 5,67 · 10-8 J/(sec•m²·K*), t is time (sec), the emissivity of the skin e = 1. 6. The amount of heat that the human body receives from solar radiation is assuming that the full 1150 eAcose intensity of solar radiation reaches the surface Q = Here A is the skin area of the person (m²), 0 is the angle of incidence of sunlight, and e is the emissivity of the skin. The emissivity of the skin in the wavelength region of solar radiation depends on the pigmentation. Dark skin absorbs about 80% of the radiation, and light skin absorbs about 60%. 6. The rate of the heat transfer W = 2