The influence of the grain size of metallic surfaces on nucleate boiling was investigated experimentally. Surfaces of electrodeposited copper, nickel, brass and chromium were studied with particular emphasis on copper (146 grains per inch to 1678 grains per inch). The pool boiling heat tranafer studies were conducted by transferring heat produced electrically through a 6-in. diameter heating surface to a pool of boiling water contained in a glass boiler.A log-log plot of the Q / A versus AT,,, data showed a definite effect of the grain size of the surface on nucleate boiling heat transfer, the heat transferred at a given AT,,, increasing as the grain size increased.For copper plated surfaces, the Rohsenow Equation for nucleate pool boiling gave values for C., and r, which showed a linear relationship when plotted separately against the number of grains per unit length of the surface. A slightly modified form of the Rohsenow correlation has been proposed which takes into account the influence of the grain size of the surface metal for predicting boiling heat transfer. tudies of nucleate pool boiling heat transfer have generally S considered that the frcquency of bubble nucleation and the rate of buhhle growth and detachment were of pritnary importance in understanding this phenomenon (1-5). The nature of the heating surface and its effect on nucleate pool boiling heat transfer has been studied extcnsively. However, aside from general agreement on the role of the surface in providing nucleation sites, the nature of these sites is but partially understood and, as yet, it is not possible to specify a particular method of surhcc preparation to achieve a desired heat transfcr rate.Only two studics on the effect of grains and grain boundaries have bccn reported ( 8 , 7 ) and these were not conclusive. This study walr therefore undertaken to further explore the role of grains anti grain boundaries in nucleate boiling heat transfer.
The role of grains and grain boundariesAll nirtals in their usual condition are polycrystalline, consisting of grains and grain boundaries. h4cLeancg) defines the grain t)oundary in a piece of metal as the boundary separating two crystals (or grains) that differ either in crystallographic oricntatiori, composition or dimensions of the crystal lattice or in two or all of these properties. Grain boundaries are regions of misfit o r strained regions between adjacent crystalline phases. Whatever the precise structure of grain boundaries, the fact is that the atonis at the boundary between two crystals niust be displaced from positions which they would occupy in a crystal and therefore there is departure from the perfect crystal structure. Thus, each boundary atom on the average has more total energy than a grain interior atom (I)), and the grain boundaries as a wholc On a itudii expirimentalement l'influence des dimensions des grains des surfaces mhtalliques sur l'ibullition germinie. Le travail a port6 sur des surfaces de cuivre, nickel, laiton et chrome diposis par voie galvanique, mais particul...
