Combined Radiation And Convection

Algebra Business Calculus Education Mathematics Technology

EXPERIMENT NO: 01                                                                                       DATE: 25-07-12 . COMBINED RADIATION AND CONVECTION . . AIM • To determine the heat transfer due to radiation and convection from cylinder. • To find emissivity at different temperatures. • To find natural convection coefficient. . APPARATUS REQUIRED • Thermometer • Heater • Stopwatch • Sand paper . THEORY If a surface, at a temperature above that of its surroundings, is located in stationary air at the same temperature as the surroundings then heat will be transferred from the surface to the air and surroundings. This transfer of heat will be a combination of natural convection to the air (air heated by contact with the surface becomes less dense and rises) and radiation to the surroundings. A horizontal cylinder is used in this exercise to provide a simple shape from which the heat transfer can be calculated. Note: Heat loss due to conduction is minimised by the design of the equipment and measurements mid way along the heated section of the cylinder can be assumed to be unaffected by conduction at the ends of the cylinder. Heat loss by conduction would normally be included in the analysis of a real application. . In the case of natural (free) convection the Nusselt number Nu depends on the Grashof and Prandtl numbers and the heat transfer correlation can be expressed in the form: Nusselt number, Nu = f (Gr, Pr) Rayleigh number, Ra = (Gr Pr) The average heat transfer coefficient for radiation Hrm can be calculated using the following relationship:                                                  (function(){function i(e){seraph_pds.View.InitFormulas();}if(seraph_pds && seraph_pds.View)i();else document.addEventListener(‘DOMContentLoaded’,i);})()Hrm= ΣζF(Ts4−Ta4)(Ts−Ta).. The average heat transfer coefficient for natural convection Hcm can be calculated using the following relationship:                                                   Tfilm= Ts+Ta2..                                                         β= 1Tfilm..                                                      Ngr=  gβ(Ts−Ta)D3υ2..                                                  Rad = (Grd × Pr)                                                 Num = b(Rad)n, (c and n can be obtained from the table below)                                                     Hfm= kNumD.. Note: k, Pr, and n are physical properties of the air taken at the film temperature Tfilm. The actual power supplied to the heated cylinder, Qin = V×I Table for c and n values for natural convection on horizontal cylinder. Rad c N 10-9 to 10-2 0.675 0.058 10-2 to 102 1.02 0.148 102 to 104 0.850 0.188 104 to 107 0.480 0.250 107 to 1012 0.125 0.333 . . . Alternatively a simplified equation may be used to calculate the heat transfer coefficient for free convection :                        Hcm=1.32× (Ts−Ta).25D.25.. The value for Hcm should be calculated using both the original and simplified equations and the values compared. . EXPERIMENTAL PROCEDURE…

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