Heal transfer coefficient cylinders

Fur heal transfer purposes, a standing man can be modeled as a 30 Cm diameier, 170 cm-loag vertical cylinder with both the top and botiom surfaces insulared and with the side surface at an average temperature of 34"C. For a convection heat transfer coefficient of 15 W/m "C, deiemiine the rale of heat loss from ihis man by convection in still air al 20°C. What would your answer be if the convection heal transfer coefficient is Increased to 50 W/m °C as a result of winds What is the wind-chill factor in this case Ansuers 336 W, 11 0 W, 3 .7 C... 

Consider a plane wall of thickness 2L, a long cylinder of radius r , and a sphere of radius r, initially at a nnifonn temperature T,-, as shown in Fig. 4—11. At time t = 0, each geometry is placed in a large medium that is at a constant temperature T and kepi in that medium for t > 0. Heat transfer lakes place between these bodies and their environments by convection with a uniform and constant heal transfer coefficient A. Note that all three ca.ses possess geometric and thermal symmetry the plane wall is symmetric about its center plane (,v = 0), the cylinder is symmetric about its centerline (r = 0), and the sphere is symmetric about its center point (r = 0). We neglect radiation heat transfer between these bodies and their surrounding surfaces, or incorporate the radiation effect into the convection heat transfer coefficient A. 

Variation of the local heal transfer coefficient along the circumference of a circular cylinder in cross flow of air (from Giedl, 1949). 

The discussions above on the local heat transfer coefficients arc insightful however, they are of limited value in heal transfer calculations since the calculation of heat transfer requires the average heat transfer coefficient over the entire. surface. Of the several such relations available in the literature for the average Nusselt number for cross flow over a cylinder, we present the one proposed by Churchill and Bernstein ... 

Overall heat-transfer coefficient for finned tubes. We consider here the general case similar to Fig. 4.3-3b, where heat transfer occurs from a fluid inside a cylinder or tube, through the cylinder metal wall A of thickness Ax, and then to the fluid outside the tube, where the tube has fins on the outside. The heat is transferred through a series of resistances. The total heat q leaving the outside of the tube is the sum of heal loss by convection from the base of the bare tube q, and the loss by convection from the fins, y. 

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