Heat pipe cooling of large electric motors

The condenser heat transfer of the incl tned rotating heat pipe which was less analyzed are investigated. A physical model is creatively proposed consisting of a laminar filmwise condensation region amd a bottom condensation flow region. The theoretical formulas of the average Nusselt number and the local Nusselt number are obtained by solving the flow differential equations. The effect of centrlfiigal acceleration on condenser heat transfer characteristics is also investigated. The relations between the rotational speed and average condenser heat trsmsfer coef. and local condenser heat transfer coef. are shown. There are good agreement between theoretical and experimental results, especially in high rotational speed range. HGMBNCLATURB a centrlf-'agal acceleration at tube axis A cross sectional area of bottom flow Cp specific heat of condensate d tube diameter f fanning' 3 friction factor F force on an element of bottom flow h heat transfer coefficient 1 condenser section length L latent heat of condensation Q volumetric flow rate of bottom flow i^ tube radiusR radius of rotation Ts— temperature of satxirated vapor Tw condenser wall temperature u local clrc\amferential velocity component of condensate film mean velocity of bottom flow V IocslI aucial velocity component of condensate film X distance along tube axis z distance normal to tube Interior surface a angle between heat pipe ajcls and rotation axis f momentum correction factor J thicknessof condensate film X heat conductivity of condensate p —• density of condensate y kinematic viscosity of condensate Q angle of bottom flow level ^ co-ordinate in circumferential direction G = ctga Ga = asinrtd-'/v'' H = Cp(T3-Tw)/L Nu local nusselt number = hd/A = =. (2GaPrG/(3HZ))* Pr prandtl number =(*v>Cp/A Re Helnolds number = (Ga/2 )^(' /(9 ) X dimensionless axial distance = Gx/r. Z dimensionless condensate film thickness = (2GaPrG/(3H))(^/d)* INTRODUCTION For promoting the energy conservation by utilizing the waste heat from factories, the study and development of efficient heat recovery systems which make use of heat pipe have been promoted. It is well known that heat pipe heat exchanger has a great deal of advantages for gas to gas heat exchange , such as its excellent heat transfer performance and simple structure , so its practical applications have been made most progressively till now. However, in highly fouling environment such as heavy oil flue gas, it becomes am important subject of study that the dust accumulation make heat transfer performance degeneration, one of the best way to solve this problem is to use rotating heat pipe heat exchajiger which has a rotating heat pipe bundle. According to the relative position between the heat pipe axis and the axis of rotation, rotating heat pipes can be levided into three basic groups, such as, cc-axial. parallel, and inclined. Performance characteristics of a co-axial or parallel rotating heat pipe has been described in previous literatures, but only little is known about the performance of a inclined rotating heat pipe. The longitudinal axis of the inclined rotating heat pipe is inclined to the axis of rotatlon(Fig. 1 . ) , which depends on the component of centrifugal force to pump the condensate from the condenser to evaporator to complete the cycle. The purpose of this study was to Investigate theoretically the condeaaer heat transfer characteristics of a inclined rotating heat pipe, with a new physical model, discuss the condenser heat transfer mechanism according to the flow patterns of working liquid which was shown in physical model. BXPBHIMENTAI, EQUIPMENTS The appaxatua used for this study was similar to the one used in earlier study CI ] . Fig.1. shows a schematic diagram of the overall experimental equipment. Fig.1 Schematic diagraun of experimental equipment The heat pipe was rotated using a variable speed motor, and heated electrically, and cooled by water. Further details of the experimental equipment are provide in reference ( 1 ] . OPERATION PAHAMETER3 heat Dipe length 1500 mm internal diauneter 19 mm external diauneter 22 mm condenser length 410 mm evaporator length 820 mm min. eccentricity(B. ) 500 mm working fluid water heat pipe wall coppe r max. power transmitted by each pipe 3 lev THEORETICAL PROGRAM For condensation cylindrical., inclined ces acting on the cond over the tube surface force, the centrifugaJ. steam flow velocity is along the vapor-liquid ation cases, the flow Fig. 2. within a non-capillary, rotating heat pipe,forensation film formed are the gravitational force and , when the large, shear force interface. In the operpattems aa shown by filMf., To discuss more detail, the patterns will be defined aa follows: a) condensate with the film flows along the tube interior STirface as shown by the broken line in Fig. 2; b) a bottom flow represented by the hatch area in Fig. 2, this bottom flow region, in general, blocks most of heat flow across itself and the tube surface covered by it becomes ineffective as a condensatlci heat transfer surface. Therefore, when calculating the overaJ.1 condenser heat transfer coef. for the whole condenser section, it is not necessary to take into accout the bottom flow over the tube surface. Here we only analysed the behavior of the bottom flow. Condensate film flow Analysis waa made under assumptions similar to those of Nusselt's well-known basic condensation theory. Otherwise, we assume the effects of vapor shear and gravity are small enough to be negligible. Prom Fig. 2 we see, on the condensate act the circumferential component of centrifugal acceleration, acoaasin*, in the i^-direction (acosAsin^ can be found from geometrical consideration), and the longitudinal component of centrifugsLl acceleration, asinof, in x-direction. The momentum equations in the x-directlon and ^-direction become: