Experimental investigation of the film boiling heat transfer in He II: Heat transfer coefficient

He II film boiling is of both academic and applied interests. However, information about He II film boiling is still inadequate and further study is needed from both the technical application and the scientific aspects. In the present study, a thin stainless steel foil heater (10 μm thick) is employed to induce boiling in He II. The average heater surface temperature is measured to evaluate the heat transfer performance of He II film boiling under different thermal conditions. And meanwhile, the pressure and the temperature oscillations induced by the film boiling are also measured. It is found that the pressure oscillation and the temperature oscillation highly correlate with each other, which indicates that the vapor bubble is vibrating on the heater surface during film boiling. The heat transfer coefficient of the film boiling depends on both the pressure over the heater surface and the He II bath temperature. The heat transfer coefficients of three kinds of boiling states: noisy film boiling, transition boiling and silent film boiling, are measured in the present study. The visualization of the boiling process is also carried out.     

Singularities of Realization of Film Boiling on Wire Heaters. Organic Liquids

The singularities of realization of the mode of film boiling on wire heaters are investigated in a wide range of organic liquids. Carbon tetrachloride and various alcohols (C1–C5) are used for investigation. It is demonstrated that chemical reactions accompanied by gas liberation proceed in the cavity of film boiling, which are caused by the thermal decomposition (pyrolysis) of the original substance. The kinetics of chemical gas liberation are studied, and the composition of reaction products is determined as a function of the heater and liquid temperature and of the heater material. It is demonstrated that the gas liberation in subcooled liquids at high heater temperatures is fully defined by the chemical processes. It is found that, in the case of a short heating element and in liquids subcooled to the saturation temperature, the heat transfer in the film mode occurs in the form of a self-oscillatory process with the oscillation amplitude of the heater temperature of 350–400°C. The mechanism of the emergence of such oscillation is suggested.     

Heat transfer in a subcooled evaporating liquid film

An investigation is performed of heat transfer in films of water and FC-72 liquid falling down a 60×120 mm heater. Heat transfer mode maps are constructed. Zones of structure formation and regions of emergence of breakdown of liquid film are identified, as well as regions of boiling in jets. An averaged coefficient of heat transfer was used in analyzing the experimental data. It is demonstrated that thermocapillary forces have a complex and ambiguous effect on heat transfer. The emergence of extensive stable dry spots causes a decrease in the average coefficient of heat transfer. On the other hand, an increase in the path length of film and in the amplitude of wave motion leads to the washing out of the dry spots and to an increase in the relative intensity of heat transfer. The regularities of heat transfer in the region of flow of film with breakdowns for weakly and intensely evaporating liquids differ significantly. An enhancement of heat transfer is observed under conditions of significant evaporation during structure formation in the thermocapillary mode.     

Boiling heat transfer from a horizontal flat plate in a pool of liquid hydrogen

Heat transfer from a flat plate facing upward immersed in a liquid hydrogen pool was measured for the pressures from atmospheric to 1.1 MPa. The flat plate heater used was 10 mm in width, 100 mm in length and 0.1 mm in thickness. Critical heat fluxes (CHFs) in saturated boiling increased with the increase in pressure up to around 0.3 MPa and then decreased with further pressure increase. The CHFs under subcooled condition at each pressure increased with the increase in sub-cooling. Discussions were made on the experimental results by comparing with those of the other cryogenic liquids and also the Kutateladze’s equations under saturated and subcooled conditions. The experimental CHFs were much smaller than the Kutateladze’s equation for higher pressure up to critical. The heater surface temperature was found to reach the critical temperature before the occurrence of hydrodynamic instability and jump to the film boiling regime at the lower heat flux in the higher pressure range. It was suggested that the CHFs are determined not by the heat flux but by the temperature in the higher pressure range.     

Thermo-fluid dynamics of several film boiling modes in He II in the pressure range between atmospheric pressure and saturated vapor pressure

Four film boiling modes including the silent film boiling and the noisy film boiling were discriminated experimentally. Each mode was classified through visual observation and transient pressure and temperature measurements near the heater. It was found that in subcooled He II there were two film boiling modes, which are the strongly subcooled and weakly subcooled film boiling modes. The variation of boiling state between these two modes could be visually observed well by use of a transparent heater. All mode of film boiling is clearly mapped in diagrams as a function of pressure, temperature and heat flux. It is elucidated that the existence of He I layer influences the development of the vapor layer.     

Experimental study of temperature distribution in the vicinity of film boiling

The transition of nucleate to film boiling on a flat surface is studied experimentally. In the vicinity of the boundary of a propagating center of film boiling, the distributions of the temperature, of the heat flux to liquid, of the heat-transfer coefficient, and of the velocity of motion of isotherms along an exothermal surface are obtained. The experiments are carried out in liquid nitrogen. A sapphire plate with platinum temperature microsensors deposited on it by sputtering is used as a heater. The instability of the boundary of the change of the boiling modes due to the Taylor instability of the interface is found in the film boiling region, as well as an intermediate region between film and nucleate boiling, where the heat fluxes are almost three times the critical value.     

Partial Nucleate Pool Boiling at Low Heat Flux: Preliminary Ground Test for SOBER-SJ10

Focusing on partial nucleate pool boiling at low heat flux, SOBER-SJ10, one of 27 experiments of the program SJ-10, has been proposed to study local convection and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well characterized flat surface in microgravity. An integrated micro heater has been developed. By using a local pulse overheating method in the experimental mode of single bubble boiling, a bubble nucleus can be excited with accurate spatial and temporal positioning on the top-side of a quartz glass substrate with a thickness of 2 mm and an effective heating area of 4.5 mm in diameter, and then grows under an approximate constant heat input provided by the main heater on the back-side of the substrate. Ten thin film micro-RTDs are used for local temperature measurements on the heating surface underneath the growing bubble. Normal pool boiling experiments can also be carried out with step-by-step increase of heating voltage. A series of ground test of the flight module of SOBER-SJ10 have been conducted. Good agreement of the measured data of single phase natural convection with the common-used empirical correlation warrants reasonable confidence in the data. It is found that the values of the incipience superheat of pool boiling at different subcooling are consistent with each others, verifying that the influence of subcooling on boiling incipience can be neglected. Pool boiling curves are also obtained, which shows great influence of subcooling on heat transfer of partial nucleate pool boiling, particularly in lower heat flux.     

Mathematical simulation of transient processes in a circular channel with granular layer under conditions of subcooled water boiling. Formation of pressure wave

The process of formation and growth of vapor bubbles until the instant of their merging into a vapor film is investigated in view of actual roughness of the surface and of the dependence of activation time of the respective nucleation sites on the surface temperature under conditions of pulsed heat release in the heater wall. A microlayer model is employed in view of formation of dry spot under the bubble. Also solved is the problem on the formation of a pressure wave in a circular channel as a result of explosive boiling of superheated microlayer under the bubbles. The obtained results enable one to describe a number of experimental data and investigate unsteady-state wave processes occurring under subcooled liquid boiling in the presence of a granular layer.     

Thermocapillary effects on low-g Pool Boiling from microheater arrays of various aspect ratio

Pool boiling heat transfer measurements from heaters of varying aspect ratio were obtained in low-g (0.01 g ± 0.025 g) and high-g (1.7 g ± 0.5 g) using the KC-135 aircraft. The heater aspect ratio was varied by selectively powering 2×2, 2×4, 2×6, 2×8, and 2×10 arrays of heaters in a 10×10 heater array containing individual heaters 700×700 μm² in size. Control circuitry was used to maintain a nominally isothermal boundary condition on the heater surface while the power dissipated by the heater was measured. Steady-state boiling data in low-g and high-g were obtained using FC-72 as the working fluid at two bulk fluid temperatures. At high wall superheats, boiling performance appears to decrease with an increase in aspect ratio. Strong thermocapillary convection was observed for a negligible gas concentration in the liquid. CHF was not observed for the heater used in this study indicating that CHF in low-g may be highly dependent on the surface characteristics of the heater in addition to the heater size.     

Self-organization of critical fluctuations and 1/f spectra under conditions of critical boiling

The results are given of an experimental investigation of the dynamics of vaporization under conditions of changing modes of water boiling on a platinum wire heater and with local Joule heating of aqueous electrolytic solutions. It is found that thermal fluctuations with spectral characteristics inversely proportional to frequency (flicker or 1/f noise) are generated under conditions of film boiling on a vertically oriented heater in a wide range of loads. In the region of loss of stability of film boiling, frequency spectra vary in inverse proportion to the square of the frequency. In experiments in Joule heating of aqueous electrolytes, the flicker noise is observed in the region of changing modes of vapor generation. The experimental results are indicative of the self-organization of the critical behavior of the system in the case of nonequilibrium phase transitions.     

Effect of cylindrical heater dimensions on critical heat fluxes during boiling of cryogenic liquids

Experimental data on critical heat fluxes during cryogenic liquid boiling (helium and nitrogen) on cylindrical heaters are presented for a wide pressure range (45–760 mm Hg). The effect of heater dimensions (diameter, length and wall thickness) on critical heat fluxes is studied. The data on the relation between this effect and heater material and orientation are obtained.     

The effect of mutual location of heaters on the falling film dynamics

Thin nonisothermal liquid film flowing down under action of gravity is considered. Investigation of the influence of the spanwise and streamwise arrangement of the rectangular heaters on 3-D structures, occurring at the film surface, is the main objective of the present work. Three-dimensional time-dependant mathematical model for calculation of gas-liquid interface deformations and evolution of temperature fields was developed. Our numerical investigations have shown that interaction, imposition and mutual influence of the 3D structures (bumps, lateral waves ...) takes place. In the case of streamwise arrangement of the heaters film rupture is most likely on the second heater. There is a critical backlash between the heaters, at which film deformations, including film thinning, are the largest. For the spanwise arrangement of the heaters distance between them practically do not effect on the minimum film thickness, but mutual imposition of the lateral waves and film thickening exists.     

Bubble spreading during the boiling crisis: modelling and experimenting in microgravity

Boiling is a very efficient way to transfer heat from a heater to the liquid carrier. We discuss the boiling crisis, a transition between two regimes of boiling: nucleate and film boiling. The boiling crisis results in a sharp decrease in the heat transfer rate, which can cause a major accident in industrial heat exchangers. In this communication, we present a physical model of the boiling crisis based on the vapor recoil effect. Under the action of the vapor recoil the gas bubbles begin to spread over the heater thus forming a germ for the vapor film. The vapor recoil force not only causes its spreading, it also creates a strong adhesion to the heater that prevents the bubble departure, thus favoring the further spreading. Near the liquid-gas critical point, the bubble growth is very slow and allows the kinetics of the bubble spreading to be observed. Since the surface tension is very small in this regime, only microgravity conditions can preserve a convex bubble shape. In the experiments both in the Mir space station and in the magnetic levitation facility, we directly observed an increase of the apparent contact angle and spreading of the dry spot under the bubble. Numerical simulations of the thermally controlled bubble growth show this vapor recoil effect too thus confirming our model of the boiling crisis.     

The effect of external perturbations on the energy of temperature pulsations in a heated liquid film

The dynamics of temperature pulsations on the surface of a heated water film flowing vertically down was analyzed under the conditions of 3D wave movement for an unperturbed film and upon introduction of external perturbations with “the most dangerous wavelength.” It was established for the first time that artificial perturbations increase the amplitude and spectral energy, as well as expand the frequency range of temperature pulsations in the bottom part of the heater compared to the case without perturbation. External perturbations were found to cause an increase in the average integral energy of temperature pulsations per unit time in the bottom part of the heater and, consequently, an increase in the film resistance to rupture.     

Thermal Response of Transparent Silver Nanowire/PEDOT:PSS Film Heaters

Thermal response behavior of transparent silver nanowire/PEDOT:PSS film heaters are intensively studied for manipulating heating temperature, response time, and power consumption. Influences of substrate heat capacity, heat transfer coefficient between air and heater, sheet resistance and dimension of Ag nanowire film, on the thermal response are investigated from thermodynamic analysis. Suggestion is given for practical applications that if other parameters are fixed, Ag nanowire coverage can be utilized as an effective parameter to adjust the thermal response. The heat transfer coefficient plays opposite roles on thermal response speed and achievable steady temperature. A value of ≈32 W m^?2 K^?1 is obtained from transient process analysis after correcting it by considering heater resistance variation during heating tests. Guidance of designing heaters with a given response time is provided by forming Ag nanowire film with a suitable sheet resistance on substrate of appropriate material and a certain thickness. Thermal response tests of designed Ag heaters are performed to show higher heating temperature, shorter response time, and lower power consumption (179 °C cm² W^?1) than ITO/FTO heaters, as well as homogeneous temperature distribution and stability for repeated use. Potential applications of the Ag heaters in window defogging, sensing and thermochromism are manifested.     

Control over the formation of structures in a heated liquid film

An experimental investigation is performed of the impact made by artificial disturbances on the formation of structures under conditions of flow of water film on a vertical plate with a heater. The fluorescent method is used for measuring the film thickness. It is found that artificial disturbances on the surface of liquid film may cause a significant variation of the spacing between jets, and the scenario of evolution of wave pattern depends on their intensity. Artificial disturbances on the film surface cause the spacing between jets to decrease from values corresponding to the thermocapillary-wave mode of formation of jets to values close to those of spacings typical of the thermocapillary mode. In so doing, the spacing between jets may be varied at higher values of Reynolds number than is the case in the thermocapillary mode. It is demonstrated that it is possible to control the formation of structures on the surface of a heated film of liquid, and the conditions of maximal effect of artificial disturbances are determined.     

Shear-Driven Flow of Locally Heated Viscous Liquid Film in a Minichannel

This paper considers a flow of a liquid sheared by gas in a flat mini-channel with two identical heaters arranged in a row one after another in a streamwise direction at the bottom wall. The present study is focused on the investigation of influence of local heaters arrangement and size on thermocapillary deformations in a viscous film, gravity effect is also investigated. 3D one-sided model is considered, viscosity of the liquid is supposed to be temperature dependent. Numerical analysis reveals that interaction and mutual influence of 3D structures takes place. Film pattern changes qualitatively depending on the heaters arrangement and form. For rectangular heaters a middle stream exists. Minimum film thickness value increases and its location moves to heater edges for rectangular heaters. A critical backlash between two heaters, at which film thinning is the largest, exists. Gravity significantly affects on the film deformations. Decreasing of gravity level leads to a flow destabilization and film deformations, especially film thinning, essentially increases.     

Correlation of time dependent recovery from film boiling heat transfer in He II

A correlation is presented that describes the behaviour of time dependent recovery from film boiling in He II. In a one dimensional heat transfer experiment, the recovery time from the film boiling once heat generation stops is observed to be a function of the energy applied to the heater during film boiling. This correlation has a power law dependence which can be physically understood in terms of heat capacity of the heat transfer sample and the film boiling heat transfer coefficient. A direct comparison of experimental data with the analysis is achieved by adjusting the value of the transient film boiling heat transfer coefficient. Data can be predicted to within 20% for recovery under SVP conditions. The results are somewhat less certain for data taken in subcooled He II.     

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm². The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached. Pis’ma Zh. Tekh. Fiz. 25, 39–46 (November 12, 1999)     

Experimental Study of the Relation Between Heat Transfer and Flow Behavior in a Single Microtube

The flow boiling heat transfer in microchannels have become important issue because it is extremely high-performance heat exchanger for electronic devices. For a detailed study on flow boiling heat transfer in a microtube, we have used a transparent heated microtube, which is coated with a thin gold film on its inner wall. The gold film is used as a resistance thermometer to directly evaluate the inner wall temperature averaged over the entire temperature measurement length. At the same time, the transparency of the film enables the observation of fluid behavior. Flow boiling experiments have been carried out using the microtube under the following conditions; mass velocity of 105 kg/m² s, tube diameter of 1 mm, heat flux in the range of 10 ~ 380 kW/m² s, and the test fluid used is ionized water. Under low heat flux conditions, the fluctuations in the inner wall temperature and mass velocity are closely related; the frequency of these fluctuations is the same. However, the fluctuations in the inner wall temperature and heat transfer coefficient are found to be independent of the fluctuation in the mass velocity under high heat flux conditions.