Numerical Studyon Natural Convection Heat Transfer Of Molten Salt Around Circular Tubes

Natural convection heat transfer is one of a primary and important heat transfer ways in thermal energy storage tanks which are full ofhigh temperaturemolten salts and used in many commercial solar thermal power plants.In this paper,by means of numerical simulation,under the condition of two-dimensional,laminar flow and steady state,the heat transfer of single tube and row of tubes are studied in large space and limited space.From the investigation of the natural convection on the surface of a single tube within large space,it shows that the numerical value of the unconfined tube is consistent well with the Fand's correlation.On the basis of this result,the impact of upper-wall on the buoyant plume generated above the heated tube is also studied to observe the natural convection heat transfer characteristics.It shows that when the natural convection of Ra?2×10~4,the upper-wall has almost no effect on the heat transfer of heated tube when h/d?3.To study the heat transfer characteristicsof the upper heated tube about the four staggered tubes bank in large space,the dimensionlesshorizontal and vertical tube pitches are alteredin a certain range.In order to ensure the upper-tube's heat exchange more fully,the D_v and P_vmust be more than 2;Considering the heat transfer of the upper-tube,the best tube pitch is D_v=4 and P_v=8.Then,three columns heated tubes are also explored in large space,the heat exchange efficiency of these tubes in the middle column varies almost linearly.Affected by velocity influence and thermal effect of the thermal plume,theaveraged Nusselt number of the second tube is higher than others.At last,four heated tubes with large diameter arrange in aligned tubes bankand staggered tubes bank in limited space.The effects of the location of the heated tubes and the Rayleigh number on the heat transfer and fluid flow in the cavity are studied,too.The results shows thatthe stability of flow in molten salt media and thesymmetry of temperature field distribution are both worse with the increasing of Rayleigh number andtube spacing;When the heated tubes decorate in staggered way,the heat transfer effect of the upper-tube is worse with the increase of?,however,arranged in aligned way,the heat transfer effect of the upper-tube is betterwith the increase of?.