This paper deals with investigation results on boiling up and crisis phenomena for nonstationary heat release in falling liquid films. According to the experimental results, in the studied range of irrigation degree alteration (Re in = 50–1300), parameters, characterizing decay of the falling liquid film with stepped heat release (distribution of time of boiling incipience along the liquid film, velocities of movable boundaries in the boiling-up and drying fronts), depend complexly on the Reynolds number, wave characteristics and heat flux density. Experiments were carried out with the use liquid nitrogen. Step-wise heat release was supplied on the vertical plane constantan foil of the 25-mkm thickness and 40-mm length. When loading impulses of high intensity, film decay is determined by dynamic characteristics of propagation of the self-maintained fronts of evaporation and the complex shape of structures, formed during its development. The effect of heat flux density on the time of boiling-up expectation and structures of evaporation fronts is shown for different Reynolds numbers. The experimental data obtained on the average propagation velocity of the self-maintained front of evaporation are compared with the simulation model results.

Studies on two-phase flow in small scale pipes have become more important, because of the application of mini-scale devices in several engineering fields including, high heat-flux compact heat exchangers, and cooling systems of various types of equipment. In a mini pipe the behavior of two phase flow is not the same as flow in conventional pipes. The difference is caused by different effective forces; for e. g. inside a mini pipe capillary forces are more important in comparison with gravitational forces. This paper is devoted to numerical simulation of gas-liquid two phase flow in a vertical mini pipe. Prediction of bubble shape and the effects of gas and liquid velocities on flow characteristics are considered. Also simulation involves prediction of changes in average void fraction along pipe axis. Numerical simulations in this paper are performed by a designed and developed CFD package which is based on Eulerian-Eulerian approach. The governing equations which are solved in the CFD package are momentum, continuity and Fractional Volume of Fluid (VOF) function equations. The fluid is assumed to be viscous and incompressible. The pressure-velocity coupling is obtained using the SIMPLEC algorithm. The geometry, which have been studied in this paper, is a D = 1.02 mm pipe, with 500 mm height. Bubble shape and the distribution of void fraction in a mini pipe are related to many parameters such as: gas and liquid velocities, pressure losses and etc. Since these mechanisms vary over time, time-average value of void fraction is used. Comparisons between Numerical results and experimental work which performed by hibiki et al. [1] indicated good agreement. Also results have shown that the present model is capable to simulate the behavior of nitrogen-water two phase flow in a mini pipe with acceptable accuracy. Furthermore, the results indicates that average void fraction along the pipe axis is related to the height and nitrogen superficial velocity. Also it is observed that at constant nitrogen superficial velocities, average void fraction decreases with water superficial velocity increments.

A nitrogen double expander cycle has been widely used for liquefaction of natural gas in LNG-FPSO (Floating Production, Storage, and Offloading). An aluminum plate-fin heat exchanger (ALPHE) is usually adopted in the liquefaction cycle. In general, the ALPHE has a very large heat transfer surface area per unit volume. This surface area consists of primary and secondary (finned) surfaces. Even taking into account fin efficiency of the secondary surface, the effective surface area per unit volume can be typically five times greater than that of a shell-and-tube heat exchanger. Various types of fin are available in ALPHE and the fin type should be selected properly to optimize the performance. For example, serrated, wavy and perforated fin are particularly suitable for gas streams. The selection and design of the layer arrangement and effective length of each stream are very important design parameters for ALPHE. In this paper, to optimize the design of ALPHE, the effects of the design parameters on the performance of ALPHE were studied using a simulation method. The properties of nitrogen and natural gas were calculated from proper equations of state. Because the performance of ALPHE is mainly influenced by the fin type, fin frequency, fin height, fin thickness, and layer arrangement, the effects of the geometric design parameters on the performance of ALPHE were studied, and the optimum design conditions were suggested in this paper.

Characteristics of heat transfer and hydrodynamics of boiling of liquid nitrogen on the surfaces with different types of non-uniformities at the presence of external electric fields are experimentally investigated. It is shown that the formation of field traps is a major mechanism of heat transfer enhancement. And this effect result in noticeable change of two-phase hydrodynamics in vicinity of heated surface.

During the last decade a number of studies of boiling heat transfer in carbon dioxide notably increase. As a field of CO 2 practical using corresponds to high reduced pressures, and a majority of available experimental data on CO 2 flow boiling even in submillimetric channels relate to turbulent liquid flow regimes, a possibility arises to develop sufficiently general method for HTC predicting. Under the above conditions nucleate boiling occurs up to rather high flow quality, even in annular flow regime due to extremely small size of an equilibrium vapour bubble. This conclusion is in agreement with the available experimental data. The predicting equation for nucleate boiling heat transfer developed by one of the present authors in 1988 is valid for any nonmetallic liquid. A contribution of forced convection in heat transfer is calculated according to the Petukhov et al. equation with correction factor, which accounted for an effect of velocity increase due to evaporation. This effect can be essential at relatively small heat fluxes and rather high mass flow rates. The Reynolds analogy and homogeneous model are used in order to account for the convective heat transfer augmentation in two-phase flow. Due to low ratio of liquid and vapour densities at high reduced pressures the homogeneous approximation of two-phase flow seems to be warranted. A total heat transfer coefficient is calculated as an interpolated value of boiling and convective HTCs. The experimental data on CO 2 flow boiling related to regimes before heated wall dryout incipience are in rather good agreement with the calculations. Besides the data on carbon dioxide flow boiling, the results on water, helium, nitrogen and some refrigerants were used for comparison; at rather high reduced pressures the computed and the measured values of HTCs are in a good agreement. The data include results obtained in the channels of a diameter from 0.6mm up to 18mm. It is clear that at high reduced pressures there is no strong variation in boiling heat transfer with channel size decrease, it means that a classification on channel size has no sense if it does not consider liquid/vapour densities ratio.

The paper presents some experimental data of the cool down by means of liquid nitrogen of the components in a small facility equipped with two cold boxes connected by an horizontal transfer channel. Temperature transients, pressure and pressure drops, heat transfer and flow rates are investigated.

Nearly 80% of all women may suffer from menorrhagia caused by uterine fibroids (leiomyomas) which are benign tumors made up of muscle and fibrous tissue that grow from the muscular wall of the uterus. The vast majority of women whose symptoms are strong enough to require treatment obtain a hysterectomy. Other treatment options which are less invasive than hysterectomy include thermal therapies such as thermal ablation or cryosurgical removal of tissue. This project numerically evaluates the efficacy of a liquid-nitrogen-based cryotherapy for the treatment of uterine fibroids. A bioheat transfer model was utilized which includes both the effects of blood perfusion and the impacts of liquid-to-solid phase change. Changes in all physical properties including thermal conductivity, heat capacity, and perfusion rate were taken into account as the tissue passed through a range of temperatures where it would be transitioning from unfrozen to fully frozen. The numerical model was based on a one-dimensional unsteady bioheat equation. The results show that even for the direct-contact cooling, it is unlikely that intracellular ice would form during the procedure. On the other hand, based on data obtained from previous cell-survival studies, it was found that necrosis would occur when the cooling rates exceeded 30°/min. According to the present numerical results, necrosis would occur within the tissue up to a depth of approximately 5.8 mm, thereby ensuring that sufficient tissue would be cryosurgically destroyed to result in effective treatment.

An experimental and numerical study of a laminar boundary layer with combustion has been carried out at hydrogen and nitrogen fuel mixture blow through a porous plate. At that main flow velocity ranged from 2 to 4 m/sec and the mass fraction of hydrogen in the fuel from 1 to 11%. The lower limit of stable combustion depending on the blow intensity and hydrogen content in the fuel mixture was obtained experimentally. Data on the temperature distribution in the boundary layer have been obtained and analyzed. The simulation results show that in this range of parameters combustion occurs in the kinetic mode.

Hydrogen-fuelled internal combustion engines are still investigated as an alternative for current drive trains because they have a high efficiency, near-zero noxious and zero tailpipe greenhouse gas emissions. A thermodynamic model of the engine cycle enables a cheap and fast optimization of engine settings for operation on hydrogen. The accuracy of the heat transfer sub model within the thermodynamic model is important to simulate accurately the emissions of oxides of nitrogen which are influenced by the maximum gas temperature. These emissions can occur in hydrogen internal combustion engines at high loads and they are an important constraint for power and efficiency optimization. The most common models in engine research are those from Annand and Woschni, but they are developed for fossil fuels and the heat transfer of hydrogen differs a lot from the classic fuels. We have measured the heat flux and the wall temperature in an engine that can run on hydrogen and methane and we have investigated the accuracy of simulations of the heat transfer models. This paper describes an evaluation of the models of Annand and Woschni with our heat flux measurements. Both models can be calibrated to account for the influence of the specific engine geometry on the heat transfer. But if they are calibrated for methane, they fail to calculate the heat transfer for hydrogen combustion. This demonstrates the models lack some gas or combustion properties which influence the heat transfer process in the case of hydrogen combustion.

In recent years, many researchers investigated micro channel heat exchangers because of its high efficiency and compactness. However, few experimental studies about micro channel heat exchanger in cryogenic environment have been conducted. In this study, micro channel was fabricated by chemical etching and heat exchanger core was made by diffusion bonding method for cryogenic reliability. Performance test was conducted in cryogenic test rig. Working fluids are liquid nitrogen and methane gas. Methane gas was condensed in the micro channel heat exchanger. Heat transfer coefficients and pressure drop were measured and the heat transfer characteristics were investigated. These results can be used to design the heat exchanger of gas liquefaction plant.

This paper presents experimental results on heat transfer characteristics of turbulent gas flows though a micro-tube with constant wall temperature. The experiments were performed for nitrogen gas flows through a micro-tube with 242μm in diameter and 50 mm in length. The wall temperature was maintained at 5K, 20K and 30K higher than the inlet temperature by circulating water around the micro-tube, respectively. In order to measure heat transfer rate of gas flow through a micro-tube, the total temperature at a micro-tube exit was measured. The stagnation pressure was chosen in such a way that the Reynolds number ranges from 3000 to 12000. The outlet pressure was fixed at the atmospheric condition. The total temperature at the outlet, the inlet stagnation temperature, the mass flow rate, and the inlet pressure were measured. The heat transfer rates obtained by the present study are higher than those of the incompressible flow. This is due to the additional heat transfer near the micro-tube outlet caused by the energy conversion into kinetic energy. A correlation for the prediction of the heat transfer rate of the turbulent gas flow through a micro-tube was proposed.

The experimental investigations of non-condensable gases effect on the steam condensation inside multirow horizontal tube bundle of heat exchanger under heat transfer to boiling water were carried out at the large-scale test facility in the Institute for Physics and Power Engineering (IPPE). The experiments were carried out for natural circulation conditions in primary and secondary circuits of the facility at primary circuit steam pressure of P s1 = 0.34 MPa. The experimental heat exchanger’s tube bundle consists of 248 horizontal coiled tubes arranged in 62 rows. Each row consists of 4 stainless steel tubes of 16 mm in outer diameter, 1.5 mm in wall thickness and of 10.2 m in length. The experimental heat exchanger was equipped with more than 100 thermocouples enabling the temperatures of primary and secondary facility circuits to be controlled in both tube bundle and in the inter-tubular space. The non-condensable gases with different density — nitrogen and helium were used in the experiments. The volumetric content of gases in tube bundle amounted to ε = 0.49. The empirical correlation for the prediction of the relative heat transfer coefficient k/k 0 = f (ε) for steam condensation in steam-gas mixture was obtained.

Convective heat transfer for subcooled liquid nitrogen in a smooth horizontal pipe with internal sources is studied by analytical and numerical methods. For high Reynolds numbers the numerical results are in good agreement with standard heat transfer correlations. At smaller Reynolds numbers (<10,000), large circumferential and longitudinal temperature distributions can observed. The effect of localized heat sources on the heat transfer process is also investigated to simulate insulation failures in cryogenic pipelines. Results show that the presence of constant heat sources is detrimental to the heat transfer from both laminar and turbulent flows.

Forced convection transient heat transfer coefficients were measured for various gases (helium, nitrogen, argon and carbon dioxide gas) flowing over a twisted heater due to exponentially increasing heat input (Q 0 exp(t/τ)). The platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as the test heater. It was twisted at the center of the heater with an angle of 45 and 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q 0 exp(t/τ). The gas flow velocities ranged from 1 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate ranged from 45 ms to 17 s. The surface temperature difference and heat flux increase exponentially as the heat generation rate increases with exponential function. The heat transfer coefficients for twisted heater were compared with those of a plate heater. They are 13 ∼ 28% higher than those of the plate one. The geometric effect (twisted effect) of heater in this study shows an enhancement on the heat transfer coefficient. This is because the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. And also, it was understood that heat transfer coefficient increase with the angle of twisted heater due to swirl motion and raised turbulence intensity. Empirical correlations for quasi-steady-state heat transfer and transient one were obtained based on the experimental data.