The transient boiling heat transfer characteristics in a pool of water and highly wetting liquids such as ethanol and FC-72 due to an exponentially increasing heat input of various rates were investigated using the 1.0 mm diameter experimental heater shaped in a horizontal cylinder for wide ranges of pressure and subcooling. The trend of critical heat flux (CHF) values in relation to the periods was divided into three groups. The CHF belonging to the 1st group with a longer period occurs with a fully developed nucleate boiling (FDNB) heat transfer process. For the 2nd group with shorter periods, the direct transition to film boiling from non boiling occurs as an explosive boiling. The direct boiling transition at the CHF from non-boiling regime to film boiling occurred without a heat flux increase. It was confirmed that the initial boiling behavior is significantly affected by the property and the wettability of the liquid. The photographic observations on the vapor bubble behavior during transitions to film boiling were performed using a high-speed video camera system.
Understanding of the transient boiling heat transfer and CHF phenomena caused by increasing heat inputs in subcooling water at high pressures is necessary to predict the likelihood of a severe, design-based accident in a water-cooled nuclear reactor. The typical trend of the CHF values in relation to the heat generation rates shown with period is as follows: the CHF gradually increases to a maximum value from a steady-state CHF, and then the CHF quickly decreases to a minimum value, and finally, the CHF again increases, accompanied by a decrease in period. The steady-state CHF corresponds to the CHF due to the heat input with a period
The direct transition from a non-boiling regime such as natural convection and transient conduction regimes to film boiling without nucleate boiling was observed by Sakurai et al. [
The measured CHFs related to subcoolings for water, liquid nitrogen, and liquid helium with pressures as a parameter disagreed with the corresponding values derived from the existing correlations given by Kutateladze [
The present work is to make clear the transition phenomena to film boiling at steady and transient CHF in nonwetting and wetting liquids. The generalized phenomena for the transitions to film boiling from single-phase conduction or transient conduction and fully or insufficiently developed nucleate boiling due to exponential heat generation rates for wide range of subcoolings and pressures were investigated in water, ethanol, and FC-72, adding the photographic approach on the vapor bubble and vapor film behavior on the cylinder surface by using a high-speed video camera.
The schematic diagram of the experimental apparatus is shown in Figure
Schematic diagram of experimental apparatus.
The boiling vessel has inspection windows and is made of stainless steel; it has an inner diameter of 200 mm and a height of 600 mm. The experimental heater is made of platinum wire and is a diameter of 1.0 mm which is horizontally mounted in the vessel. The effective length of the heater between the potential taps is about 31 mm. The heater was annealed in order to maintain an even properties and its electrical resistance versus temperature relation was calibrated in water and glycerin baths using a precision double bridge circuit. The calibration accuracy was estimated to be within ±0.5 K.
The experimental heater was heated electrically by using a fast response, direct current source (max. 700 A) controlled by a digital computer so as to give a desired time function for the heat input. The average temperature of the heater was measured by resistance thermometry using the heater itself. A double bridge circuit with the heater as a branch was first balanced at the bulk liquid temperature. The output voltages of the bridge circuit, together with the voltage drops across the potential taps of the heater and across a standard resistance, were amplified and passed through analog-to-digital (A/D) converters installed in the computer. These voltages were simultaneously sampled at a constant time interval that was changed depending on the period. The fastest sampling speed of the A/D converter is 5
Experimental condition.
Parameter | Condition |
---|---|
Heater | Platinum wire |
Liquid | Ethanol, water, FC-72 |
Pressure | 101.3 kpa–1082 kpa |
Subcooling | 0 K–60 K |
Period | 0.005 s–20 s |
Figure
Illustrative time traces of heat generation rate,
Figure
Boiling heat transfer process from non-boiling to fully developed nucleate boiling.
Figure
Boiling heat transfer processes from non-boiling to film boiling under saturated condition in ethanol. Photographs shown in Figure
Figure
Vapor film behavior during direct transition to film boiling for a period of 0.1 s at a pressure of 101.3 kPa in saturated ethanol.
Figure
Vapor film behavior during transition to CHF at pressures of 101.3, 494, and 1082 kPa in saturated ethanol due to quasi-steadily increasing heat input given by exponential time function.
The critical heat flux (CHF) in saturated condition was well dependent on system pressure. The diameter of bubbles becomes small by the increase in pressure, and then the supply of liquid to a heating surface becomes better so that transitions to film boiling occurred in higher heat flux.
Figure
Vapor film behavior during transition to CHF at a pressure of 494 kPa for subcoolings of 0, 20 and 60 K in ethanol due to quasi-steadily increasing heat input given by exponential time function.
CHF was well dependent on the subcooling. As it becomes higher in subcooling, the sensible heat in order that it raises liquid temperature to the saturation temperature corresponding to the system pressure becomes more required, so that CHF becomes high. Moreover, since the diameter of bubbles becomes small, a heat input continues until the bubbles coalesce in the nucleate boiling region to form coalesced bubbles and transition to film boiling occurs. Therefore, it is thought that CHF increases as it becomes higher in subcooling.
Typical boiling heat transfer processes due to exponentially increasing heat inputs in water are shown in Figure
Heat transfer process for period
The heat transfer processes for periods of 10 s and 0.01 s at a pressure of 101.3 kPa are shown in the figure. The processes from non-boiling to film boiling are completely different from each other from period to period. It can be found that the processes up to initial boiling heat fluxes,
Figure
Vapor film behavior during transition to fully developed nucleate boiling (FDNB) for a period of 10 s at atmospheric pressure in saturated water.
Figure
Vapor film behavior during semi-direct transition to film boiling for a period of 0.01 s at atmospheric pressure in saturated water.
Figure
The relation between
In the case of CS cylinder, the
The CHF belonging to the 1st group with a longer period occurs with a fully-developed nucleate boiling (FDNB) heat transfer process. The steady-state CHFs were due to HI. For the 2nd group with shorter periods, the direct transition to film boiling from non-boiling occurs as an explosive boiling. Fukuda et al. [
Heat transfer processes during transitions to fully-developed nucleate boiling or to film boiling at atmospheric pressure in saturated FC-72 are shown in Figure
Boiling heat transfer processes from non-boiling to film boiling or FDNB at atmospheric pressure in saturated FC-72. Photographs are shown in Figures
In the case of the CS cylinder of roughness, they show the heat transfer processes to film boiling that are completely different from each other due to the exponential period. For the period of 0.02 s, the processes up to initial boiling heat fluxes,
On the other hand, in the case of the RS cylinder of roughness for the period of 10 s as shown with a dashed line, the heat transfer processes up to,
Figure
Vapor film behavior during direct transition to film boiling for a period of 0.02 s at atmospheric pressure in saturated FC-72 with the CS cylinder.
Figure
Vapor film behavior during transition to FDNB for a period of 10 s at atmospheric pressure in saturated FC-72 with the CS cylinder.
Figure
Vapor film behavior during direct transition to film boiling for a period of 10 s at atmospheric pressure in saturated FC-72 with the RS cylinder.
The vapor behavior on the horizontal cylinder during the transition from natural convection and transient conduction regimes to film boiling in water and highly wetting liquids such as ethanol and FC-72 due to various exponentially increasing heat inputs including a quasi-steadily increasing heat input to a rapid increasing one was examined by the photographs taken using a high-speed video camera.
In the case of the highly wetting liquids, the vapor film behavior during transition to fully developed nucleate boiling was just similar to that of the direct transition to film boiling. This vapor film behavior that was rapidly growing and covering the heater surface during initial boiling could never be seen in the water experiment with a quasi-steadily increasing exponential heat input. It was confirmed that the initial boiling behavior is significantly affected by the property and the wettability of the liquid. The direct transition at the CHF from non-boiling regime to film boiling one occurred without a heat flux increase for a short period within the second group while semi-direct transition occurred with one. It was certain that the direct or semi-direct transition occurs in the case of rapidly increasing heat input when the detachment of vapor bubbles without decreasing of average surface superheat is realized. The transient CHFs are clearly divided into three principal groups for the periods.