Experimental Investigation On Active Nucleation Site Density And Bubble Frequency In Narrow Channel

Boiling heat transfer in narrow channel has better heat transfer performance. The boiling heat transfer in narrow channel is widely developed in nuclear reactor, electronics, aerospace, cryogenic refrigeration and chemical industry. The mechanism of boiling heat transfer in narrow channel is different from pool boiling, more extensive works is in emergent requirement to understand the fundamental principle about flow boiling in narrow channel. In present work, experimental investigation on active nucleation site density and bubble frequency in narrow channel has been performed. The parameters effects on active nucleation site density and bubble frequency were discussed. The experimental results of narrow channels have been compared with regular channel, and the distinct differences between them have been studied as well. Based on theoretical analysis, a semi empirical and semi theoretical correlation for active nucleation site density in narrow channel has been established.In present work, water has been used as working fluid, the material of heating wall is stainless steel, which has wall roughness of 0.52μm,the surface contact angle with water is of 73.89°. Visualization research on boiling surface was carried out with high speed camera at speed of 5000 fps. Manual counting was adopted to count the continuous bubble growing in pictures. According analysis, it is found that operating parameters influence on active nucleation site density are mass flux, subcooling, heat flux,pressure and the scale of channel. According to the experiment results, it shows that:(1) In narrow channel, the mass flux has no influence on active nucleation site density; There is a certain relationship between the active nucleation site density and the heat flux, higher heat flux results higher active nucleation site density; In subcooling fluid boiling, when the subcooling is less than 15°C, with the increasing subcooling, the active nucleation site density enhanced firstly and then reduced later on, when the subcooling is above than 15°C, the effects on active nucleation site density is quite insignificant; Bubble growth occurred on nucleation sites while the pressure is below 3 bar, under same heat flux, the active nucleation site density decreased with increasing heat flux; Smaller the channel size is, higher the active nucleation site density would be, and there is more impact on active nucleation site density with increasing heat flux; The heat transfer is enhanced with the increasing active nucleation site density, and this shows that the heat transfer in narrow channel would be enhanced.(2) The relationship between active nucleation site density and superheat both in narrow channel and regular channel are subjected to same rule, namely, N∞ΔTmw,but the index of m is higher than 1 in regular channel, and it is below 1 in narrow channel. The minimum nucleation radius is estimated based on the analysis of active cavity in heterogeneous temperature distribution, when the relationship between active nucleation site density and minimum nucleation radius meets to fractal theory, the fractal dimension is blow 1, but it is above 1 in regular channel.(3) Based on theoretical analysis, a predicted correlation on active nucleation site density in narrow channel was proposed. Compared with experiment result, the error is below±25%.(4) The bubble frequency raised with increasing heat flux. Also the superheat has the same trend, but the relationship is nonlinearable, this can be explained that the radius of cavity which generate bubble is different; When subcooling is below 15°C, the bubble frequency would be enhanced with the increasing subcooling, but it would be restrained with increasing subcooling when the subcooling is above 15°C; The traditional bubble frequency correlation to estimate regular channel could not be used to predict the narrow channels.