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Pool boiling is used in various industries and play a significant role in heat transfer. So far, multiple studies have been carried out on investigating boiling and applying heat flux on the wire. In the present paper, the boiling of the coiled wire under atmospheric pressure conditions has been investigated. The fluid temperature inside the pool is considered under both constant (equal to saturation temperature) and variable temperature conditions. The value of the ring density in the coiled wire is considered to be variable. Based on the results, changing the pool liquid temperature changes bubble departure diameter and frequency. Also, increasing the density of the coil ring increases the diameter of bubbles. It has been observed that the bubbles are usually formed inside the coil, and after moving to the two ends of the coil, they leave the coil. However, by increasing the amount of heat flux and the pool liquid temperature, the size of bubbles will be larger; therefore, the bubbles must leave the coil from the empty spaces between the rings. By increasing the amount of applied heat flux, the coil was enclosed in a layer of vapor, which results in a decrease in the amount of heat transfer coefficient, and finally, a sudden increase in temperature on the wire will occur, which indicates the critical heat flux. Also, it has been observed that the critical heat flux always arises in the coil region of wire and not in the straight part of the wire.

In the present study numerical simulation of flow boiling process has been conducted for evaluation of critical heat flux conditions under the effect of different parameters (mass flux, heat flux, channel length and surface roughness). Comparison between the results of the present study and previous researches were done. The comparison shows a good agreement between the present study and previous researches. The three different turbulence models (k-epsilon, k-omega and Reynolds Stress) are considered for simulation of boiling heat transfer and CHF phenomenon. The highest accuracy of simulation is obtained by k-epsilon model. The results express that the wall temperature value of tube with adiabatic and heated boundary conditions for first and second half of the tube is lower than the wall temperature when the fluid flows only in the heated wall section. Reduction of velocity value also leads to reduction of maximum wall temperature value and CHF value. Decreasing Roughness value as an effective parameter leads to an increase in wall temperature. Maximum value of the wall temperature, after CHF point, also increases with increase in heat flux value. CHF depends on the surface roughness and rises with increasing roughness value.

The numerical simulation of subcooled flow boiling of R-113 working fluid has been done for two different nanofluids (R-113/Al2O3, R-113/TiO2) under different volume concentrations (0.5%, 1%, and 3%). The numerical results were compared with experimental results obtained by previous researchers, and the comparison shows that the numerical results are in good accordance. Nucleation site density, bubble departure frequency, and bubble departure diameter, which are three key parameters, are investigated in this study. The results express that these three parameters have the highest variation at low Reynolds numbers. The influence of different nanoparticles concentrations on the variation of the heat transfer coefficient is studied. The results indicate there is an insignificant difference between the effect of 1% and 3% concentrations on the heat transfer coefficient that means an increase of nanoparticles more than 1% concentration cannot improve heat transfer. The effect of different non-drag forces such as lubrication force, turbulent dispersion force, and lift force is also studied. Two correlations are proposed for predicting the convective heat transfer coefficient.

One of the essential industry problems is the critical heat flux (CHF) phenomenon in the flow boiling regime which leads to the temperature jumping and damaging to the systems. Increasing the vapour volume fraction decreases the heat transfer coefficient, and finally, temperature jump will occur. Also, the existence of the bumps and indent in the flow domain changes the flow pattern. In this study, by considering bumps and indent in the tube, the boiling of fluid flow in the vertical tube is discussed. For modelling and simulating the problems, the Euler-Euler model for studying the interaction of the liquid-vapour phases was used. Some models and material specifications are declared using the user-defined function (UDF) codes to the ANSYS Fluent program. The results show that the existence of bumps and indent inside the tube causes the flow of liquid phase to be less redirected in comparison to vapour phase flow due to having more momentum; therefore, at the end of the bumps in the tube, the amount of vapour volume fraction near the wall rises sharply. By increasing the flow mass flux, the vapour volume fraction at the end of bumps increases which lead to decreasing CHF value. It has also observed that if there are bumps and indents inside the tube, there will be no significant change in the liquid flow and vapour volume fraction in the other parts of the tube, as compared to the regular tube.

One of the major industry problems is the flow boiling, where reaching to the critical heat flux (CHF) condition can lead to a temperature jump and damage of the systems. In the present study, the effects of a uniform change in tube diameter on subcooled flow boiling and CHF was numerically investigated. The Euler-Euler model was used to investigate the relationship between the two liquid and vapor phases. The ANSYS Fluent code was used for simulation. According to the results, a linear increase in the tube diameter leads to increase of vapor volume fraction adjacent to the tube wall, as compared to a regular tube with a fixed-diameter, which leads to increase of the tube wall temperature due to the low value of the heat transfer coefficient. At CHF conditions, where the tube wall temperature is much higher than that in subcooled flow boiling, an increase in tube diameter may lead to higher tube wall temperature before the temperature jump, as compared to the post-jump temperature of a tube with a constant diameter. The best approach for decreasing the tube wall temperature was found to be a linear decrease in tube diameter. For the tube diameter change angles of θ <-0.0383°, tube wall temperature exhibited a decreasing trend from the inlet of the tube to its end.