Critical Heat Flux In Microchannels Research Articles

Critical heat flux at high mass velocities in a microchannel and surface tension measurements for R514a

This paper presents pioneer experimental data on critical heat flux (CHF) during flow boiling of low-pressure fluid R514a in a microchannel. The authors conducted experiments in a single smooth and horizontal microchannel, with an internal diameter of 1.1 mm and heating sections of 46 and 100 mm, a saturation temperature range of 56 to 122.7°C, and a mass velocity range of 1500 to 7500 kg/m2s. This research achieved high critical heat flux values, up to 1.85 MW/m2. The results were compared with the CHF of R123 under similar conditions and a higher CHF for R514a was observed. The performance of CHF predictive methods against the experimental data was performed. Since predictive methods for the CHF in microchannels need surface tension data, but there is a lack of experimental information about this property for R514a in the literature, this paper also presents experimental results for the surface tension of R514a and an expression for calculation of this property as a function of the temperature.

Correlations for estimating critical heat flux (CHF) of nanofluid flow boiling

Due to the different parameters affecting critical heat flux (CHF), four models were proposed in this study to estimate this parameter. The presented models were validated for mass fluxes of 100 < G [kg/m2 s] < 2500 and working pressures of 100, 400, and 800 kPa. The nanoparticles used in this study are Alumina, Diamond, ZnO, and Graphene-Oxide. Based on the correlation coefficients, the mass flux and length of the tube have the greatest effect on the increasing and decreasing of the CHF, respectively. Also, the volume fraction and the thermal conductivity of the nanoparticle have the lowest impact on the CHF. It is observed that the most M.A.E. of previous studies is to estimate the CHF in microchannels. For the convenience of using the correlations, there is no need to calculate the nanofluid properties and the critical heat flux can be predicted only based on nanoparticle properties and volume concentration of nanoparticles in the nanofluid. Furthermore, a model was also proposed, in which there is no need for nanoparticle properties and is tuned according to the type of the nanoparticles (innovative parameter: Nanoparticle Number Nn). In this study, two types of general and binary models were presented. Based on all existing experimental data, the general model has a mean absolute error (M.A.E.) of about 13%, and the binary model has an M.A.E. of 9%. The M.A.E. of the best model was observed for CHF < 400 [kW/m2] (M.A.E = 11%), and the M.A.E. for CHF > 400 [kW/m2] was 4.5%. Using the unique correlation for microchannels instead of using the Webber number coefficient, the model accuracy significantly increased.

Correlations for saturated critical heat flux in microchannels

Experimental results of the saturated-flow boiling critical heat flux (CHF) in microchannels for both multi- and single-channel configurations were obtained from the literature. The collected database contains 629 data points, covering five halogenated refrigerants, nitrogen, and water, for a wide range of operational conditions, and different microchannel dimensions. The whole database was analyzed by using five empirical correlations to verify their respective accuracies. However, none of the existing correlations could predict the entire database precisely. A saturated CHF correlation was proposed by using boiling number, length-to-diameter ratio, and exit quality. Combining with the energy balance equation, the new correlation can predict the overall microchannel database accurately on the whole. It predicts almost 97.0% of the non-aqueous data (except R12 data points located in the macro-scale region) and 94.0% of the water data within the ±30% error band.

Conditions of liquid film dryout during saturated flow boiling in microchannels

During saturated flow boiling in microchannels, two different trends of the dryout vapor quality ( x do ) have been observed experimentally: x do ∝ G - j and x do ∝ G j with j > 0 . In the literature, the first trend is experimentally observed for refrigerants and CO 2 whereas the second trend is detected so far only for CO 2 at high mass velocities. In this paper, the model of Revellin and Thome [A theoretical model for the prediction of the critical heat flux in heated microchannels. International Journal of Heat and Mass Transfer 51, 1216–1225] for the saturated critical heat flux in microchannels is used to analyze and explain this change of trend. Their theoretical model predicts three different trends of dryout vapor quality. Indeed, it is shown here that when increasing the mass velocity, the liquid film at the outlet of the channel passes through two different states: laminar flow ( Re δ < ψ = 2300 ) and transition flow ( Re δ = ψ ) . Turbulent flow ( Re δ > ψ ) has been omitted in this study since no experimental data are available for this regime. Laminar film dryout yields a decrease of the dryout vapor quality with respect to G ( x do ∝ G - j ) , transition film dryout is characterized by an increase of the dryout vapor quality with G ( x do ∝ G j ) . Furthermore, a new criterion is proposed here to identify the laminar-to-transition mass velocity, one for refrigerants and another one for CO 2 . The criterion gives the maximum value (upper limit) of the mass velocity for which the conventional microchannel correlations for the dryout vapor quality may be used. Notably, for a given pressure, the values of the laminar-to-transition mass velocities for refrigerants and CO 2 are approximately the same. This reinforces the idea that CO 2 is not a maverick fluid but just a normal fluid used at a higher reduced pressure in many boiling applications than conventionally used refrigerants.

On the Nature of Critical Heat Flux in Microchannels

The critical heat flux (CHF) limit is an important consideration in the design of most flow boiling systems. Before the use of microchannels under saturated flow boiling conditions becomes widely accepted in cooling of high-heat-flux devices, such as electronics and laser diodes, it is essential to have a clear understanding of the CHF mechanism. This must be coupled with an extensive database covering a wide range of fluids, channel configurations, and operating conditions. The experiments required to obtain this information pose unique challenges. Among other issues, flow distribution among parallel channels, conjugate effects, and instrumentation need to be considered. An examination of the limited CHF data indicates that CHF in parallel microchannels seems to be the result of either an upstream compressible volume instability or an excursive instability rather than the conventional dryout mechanism. It is expected that the CHF in parallel microchannels would be higher if the flow is stabilized by an orifice at the entrance of each channel. The nature of CHF in microchannels is thus different than anticipated, but recent advances in microelectronic fabrication may make it possible to realize the higher power levels.