Heat transfer characteristics of a two-phase closed thermosyphon using different working fluids

In this paper, an enhanced FORTRAN code was combined with the EES software to predict the vapor flow thermal resistance effects on the heat transfer characteristics of a two- phase closed thermosyphon (TPCT). Different refrigerants such as water, ammonia, R-11, R-22 and R-134a were tested. Also an enhanced time integration scheme was recommended for solving the governing equations in FORTRAN code.     

Prediction of temperature performance of a two-phase closed thermosyphon using Artificial Neural Network

Here, the temperature performance of a two-phase closed thermosyphon (TPCT) was investigated using two synthesized nanofluids, including carbon nano-tube (CNT)/water and CNT-Ag/water. In order to determine the temperature performance of a TPCT, the experiments were performed for various values of weight fraction and input power. To predict the other experimental conditions, a reliable and accurate tool should be applied. Therefore Artificial Neural Network (ANN) was applied to predict the process performance. Using ANN, the operating parameters, including distribution of wall temperature (T) and the temperature difference between the input and the output water streams of condenser section (?T) were determined. To achieve this goal, the multi-layer perceptron network was employed. The Levenberg–Marquardt algorithm was chosen as learning algorithm of this network. The results of simulation showed an excellent agreement with the data resulted from the experiments. Therefore it is possible to say that ANN is a powerful tool to predict the performance of different processes.     

Modeling analysis of bubble flow regime in a closed two-phase thermosyphon

Predictions of the operating liquid level in the evaporator of a closed two-phase thermosyphon (gravity heat pipe) are given throughout a simplified analysis which takes the influence of the dimension and condensation heat transfer in the condenser of the heat pipe into account. In order to verify the accuracy of our model comparison of the present study with some published results is made by means of computational examples.     

Heat transfer characteristics of a two-phase closed thermosyphon using de ionized water mixed with silver nano

Effect of using silver nanofluid (De Ionize water mixed with silver nano and particles less than 100 nm.) on heat transfer characteristics of a two-phase closed thermosyphon at normal operating condition was investigated in this research. The thermosyphon made by copper tube with 7.5, 11.1 and 25.4 mm ID. The filling ratios of 30, 50 and 80% by evaporator length and aspect ratios of 5, 10, and 20 (Le/d _i ) with vertical position. Pure water and DI water mixed with silver nanofluid of us as working fluid to compare. The working temperatures were 40, 50 and 60°C. It was found that, the maximum hat transfer rate of 750.81 W, with aspect ratio of 20(diameter of 25.4 mm ID) and working temperature of 60°C. The DI water mixed silver nanofluids more than approximate 70% to compare with pure water.     

Noncondensable gas effect on condensation in a separate type two-phase closed thermosyphon

An analytical study is presented of the noncondensable gas effect on vapor condensation in a separate type two-phase closed thermosyphon. The appreciable detrimental effect is observed from the results obtained by investigating condensation of steam-air and steam-hydrogen mixtures. It is shown that the effect of hydrogen on condensation is more remarkable than that of air for the same operating conditions of the thermosyphon. By examining four working temperature levels, the noncondensable gas effect is found to be accentuated at high system pressures. The computed results are all corresponding to the parameter range of engineering interest, and intended to have practical applications to designing separate type heat pipe heat exchangers.Eine analytische Studie des Einflusses eines nicht kondensierbaren Gases auf die Dampfkondensation in einem separaten, geschlossenen Zwei-Phasen-Thermosyphon wird vorgestellt. Der merklich schädliche Einfluß wird beim Untersuchen der Kondensation von Dampf-Luft- und Wasserstoff-Dampf-Gemischen beobachtet. Es wird gezeigt, daß für die gleichen Arbeitsbedingungen des Thermosyphons der Einfluß von Wasserdampf auf die Kondensation beachtlicher ist, als der von Luft. Bei der Untersuchung von vier Arbeitstemperatur-Niveaus wurde herausgefunden, daß der Einfluß des nichtkondensierbaren Gases bei hohen Systemdrücken herausragend ist. Die errechneten Ergebnisse sollen bei der Auslegung von einem separaten geschlossenen Zwei-Phasen-Thermosyphon Verwendung finden.     

Determination of the operation range of a vertical two-phase closed thermosyphon

A comprehensive model, proposed for a vertical two-phase closed thermosyphon (TPCT) by the present authors, is further developed by utilizing the criteria for dryout, flooding and boiling limits to investigate the effects of filling ratio on them together, while the available models can just consider one or two limits of them. A new concept named dryout ratio is proposed, which can be used for predicting dryout limit. The empirical correlation and the empirical value, provided by other researchers, are used for predicting flooding and boiling limit, respectively. The experiments with nitrogen as working fluid are performed, and compared with the calculations. The maximum filling ratio is introduced, beyond which the liquid could be carried to condenser and the heat transfer performance can be deteriorated. And then the closed operation range of a vertical TPCT is finally determined, which has not been reported before. The effects of operating pressure and geometries on the range are also analyzed.     

Measurements in a natural convection loop

Fluid temperature measurements are reported for a natural convection loop operating in the steady state regime. Averaged and fluctuating measurements were made for a toroidal thermosyphon oriented in the vertical plane that was heated over the lower half and cooled over the upper half. The measurements were made along the tube diameter at the entrance and exit of the heating sections for heat inputs from 1400 to 8410 w/m². The amplitude of the temperature fluctuations exceeded 10 °C near the inner walls indicating the effects from recirculating flow regions at the entrances to the heating and cooling sections.

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Messungen in einem Kreislauf mit Naturumlauf

Zusammenfassung Es wird über Messungen der Fluid-Temperatur in einem mit freier Konvektion stationär betriebenen Kreislauf berichtet. Die Messungen wurden bei leichten Fluktuationen an einem toroidalen Thermosyphon mit vertikaler beheizter Platte durchgeführt, bei dem die untere Hälfte beheizt und die obere gekühlt war. Die Temperaturenmessungen wurden über den Rohrdurchmesser am Eintritt und Austritt der beheizten Strecke für Wärmestromdichten von 1400-8410 w/m² durchgeführt. Die Amplituden der Temperaturfluktuationen überstiegen 10 K in der Nähe der inneren Wand und zeigten dabei Rezirkulationseffekte der Strömung am Eintritt des beheizten und des gekühlten Abschnitts an.

     

Unsteady natural convection in a triangular enclosure

This paper investigates the finite-difference solution of the unsteady problem of natural convection in a triangular enclosure based on the use of physical variables: velocity components, pressure, and temperature.Translated from Izvestiya Akademii Nauk SSSR, Uekhanika Shidkosti i Gaza, No. 5, pp. 169–173, September–October, 1985.     

Heat transfer enhancement using Al2O3/water nanofluid in a two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT) is a device for heat transmission. It consists of an evacuated-close tube filled with a certain amount of working fluid. Fluids with nanoparticles (particles smaller than 100 nm) suspended in them are called nanofluids that they have a great potential in heat transfer enhancement. In the present study, we combined two mentioned techniques for heat transfer enhancement. Nanofluids of aqueous Al₂O₃ nanoparticles suspensions were prepared in various volume concentration of 1–3% and used in a TPCT as working media. Experimental results showed that for different input powers, the efficiency of the TPCT increases up to 14.7% when Al₂O₃/water nanofluid was used instead of pure water. Temperature distributions on TPCT confirm these results too.     

Natural convection in a closed parallelepiped with a local energy source

An experimental and numerical analysis was performed to investigate the laminar regimes of conjugate thermogravitational convection in a closed parallelepiped with heat-conducting walls of finite thickness in the presence of a local energy source with convective heat exchange with the environment. The numerical studies were performed using the Fluent software. The experimental data and numerical results agree well each other.     

Experimental investigation of fluid convection in a closed toroidal channel

The results of investigating convective fluid motion in a closed toroidal channel are analyzed and compared with the results of calculations based on the Lorenz model. It is shown that this model adequately describes the fluid convection only in the case of a large aspect ratio of the mean radius of the torus to its cross-sectional dimension and small heating power. The measurements and visual observations record substantial deviations from the assumptions underlying the Lorenz model in the domain of problem parameters in which, in accordance with the calculations, bifurcations and instability of steady solutions of the Lorenz model should take place.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 20–28, July–August, 1995.     

Estimate of the influence of three-dimensional effects on the development of unsteady natural concentration convection in a closed region

The problem of calculating unsteady natural concentration convection of a binary mixture of gases with arbitrary finite ratio of their densities in a closed rectangular thermally insulated region has been considered in the framework of the two-dimensional formulation by Nikulin and Strelets [1]. The present paper, which continues their work, is devoted to an analysis of the influence of the three-dimensional effects associated with the finite size of the region on the characteristics of the flow.