Final Program EXPRES 2012 - Conferences

F. Heat transfer coefficientsHeat transfer coefficient for convection fromthe cover to the environment, caused by air flow(wind) hc , c / ais given by the following relation je(Duffie i Beckman [12]):A ch c , c / a5.7 3.8 (11)A rHeat transfer coefficient for the radiation ofheat between the transparent cover and the sky hr , c / sis calculated by :2 2hr , c/s c Tc Ts Tc Ts (12)Convective heat transfer coefficient betweenthe surrounding pipe layer and the transparent cover,hc , e / cmay be calculated by [14]:Te Tc Aeh, / 3.25 0 .0085(13)c e c 4re ArHeat transfer coefficient originating from theradiation form the surrounding pipe layer to thetransparent cover hr , e / cis given by [14]:A cA r2 2 Te Tc Te Tc Aeh r , e / c1 / c Ae / Ac 1 / c 1 Ar(14)Heat transfer by convection through theevacuated area between the collector pipes and thesurroundin collector layer may be neglected, so theheat transfer ceofficient by convection is hc , r / e 0 .Heat transfer coefficient for radiation from thecollector pipe to the collector surrounding layerh r r e, is given as:, /2 2Te Tr Te Tr h r , r / e1 / r Ar / Ae 1 / e 1(15)Total heat transfer coefficient U r/f between the outsidesurface of the collector pipe and the working fluid is[14] :1U r / f (16)rr , o ln r, o /r, i Ar , orhfAr , iWhere r r,o i r r,i are outside and inside radiusof the pipe of the collector , λ r heat conductioncoefficient of the collector pipe h f coefficient of heattransfer by convection from the internal surface of thepipe of the collector to the working fluid. Coefficientof heat transfer by convection, from the internal pipeside to the fluid h f , is given as:N uf fh f(17)dWhere N uf is the Nusselt number calculatedaccording to Sieder and Tate relation [16, 17]:For laminar fluid flow through the pipe Re