Natural Convective Boiling In Narrow Channels

As natural convective boiling in narrow space is frequently encountered in engineering, such as microelectronic device cooling, plate-type nuclear fuel element cooling, and nuclear safety, experimental and numerical studies have been performed for saturated R-113 at heat flux from very low to critical in narrow vertical rectangular channels under atmospheric pressure. The channels consist of a pair of flat, transparent or porcelain, and isoflux plates. The large-power isoflux heating plates for observation and measurement of temperature distribution are manufactured successfully. Onset of boiling, boiling flow pattern, outflow of upward two-phase fluid through the edges, and flow instability are carefully observed and recorded. The effect of symmetric and asymmetric heating, heating processes, gap sizes and heights of the channels, and heat flux on heat transfer is studied systematically. The results of observation and measurement illustrate that Q_ONB decreases with decrease of gap sizes of the channels and boiling flow pattern depends on gap sizes and heat flux. Maximum wall superheat does not always exist at the top of the boiling surface. Superheat decreases as the channel narrows at low heat flux, but it inverts at high heat flux. For a given pair of heating plates, there exists an optimum gap size of heat transfer enhancement. The increase of the channel height has the same effect as the decrease of gap sizes of the channels on heat transfer. The non-dimensional relationships among the rate of heat transfer, CHF, and various variables are determined respectively. From the viewpoint of " thermal drag", a numerical analysis is carried out to explore flow and heat transfer characteristics of steady natural convective boiling in vertical channels with uniform wall heat flux based on drift-flux model, especially heating effect on mass flux. Heat transfer mechanisms in narrow vertical channels are discussed and a heat transfer model is developed. The effect of gap sizes and heights of the channels and vapor pressure on CHF is theoretically investigated. The results of numerical analysis show that for a given channel, mass flux in the channel does exist a maximum, so does those of the experiment. There exists thermal drag in natural convective boiling. It plays a dominant part in case of heavy heating. The phenomenon that mass flux falls down very rapidly after its maximum might be concerned with the third type of boiling crisis.PhD Candidate: Xia Chunlin ( Thermal Science ) Directed by: Prof. Guo Zengyuan...