Heat transfer during pool boiling of subcooled pure water and water with the addition of Al
2
O
3
microparticles is considered. The presented brief literature review indicates a clearly insufficient ...experimental base, without which it is impossible to talk about a direct dependence of the critical heat flux on the concentration of particles in a liquid. The methods used in the considered publications for studying heat transfer during boiling do not give a complete picture of the entire process and do not allow one to draw unambiguous conclusions. In the present work, the main research method was gradient heatmetry. The applied heterogeneous gradient heat flux sensors made it possible to directly obtain local the heat flux (HF) and proved to be a reliable tool in the study of phase transitions in a unsteady formulation of the problem. The article considers three temperature differences, 384, 391, and 400°C, selected based on the resource of the model (time of uninterrupted operation of the model and primary converters) and in view of the stable film boiling mode characteristic of the critical the heat flux (HF). In the entire studied range of mass concentrations of Al
2
O
3
microparticles from 1 to 4%, an augmentation of heat transfer was observed during boiling of subcooled water at atmospheric pressure. The critical HF for the considered temperature differences turned out to be 3.6–5.1 times higher than for pure water. For each temperature regime, the optimal concentrations of Al
2
O
3
microparticles (particles) corresponding to the maximum HF were revealed. An increase in the critical temperature in all regimes and, as a consequence, in the Heat Flux Per Unit Area is presumably due to a change in the thermophysical properties of the suspension and the perturbations introduced by these particles into the boiling process.
Diffusion welding in gradient heatmetry Barabanova, O A; Sapozhnikov, S Z; Salmin, P A ...
Journal of physics. Conference series,
12/2021, Volume:
2144, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
Gradient heatmetry allows you to record, process and analyze pulsations of heat flux, which is of paramount importance in the research of gas flow around bodies, in the study of complex heat ...transfer, etc.
The article is about the application of the gradient heat flux measurement in the study of heat transfer during steam condensation at the inner surface of the pipe during different directions of ...water and steam supply. The installed gradient heat flux sensors (GHFS) and thermocouples did not disturb the condensate film flow on the pipe surface. Heat flux per unit areas and temperatures were measured along the length of the pipe. The obtained results confirmed the high informative of gradient heat flux measurement, as well as the capability of GHFS operating under experimental conditions.
The gradient heat flux sensor made of a bismuth single crystal is calibrated. The volt–watt sensitivity of the sensor is determined and the method of data processing based on the 1D heat conduction ...equation for a thin plate is proposed. This method is tested on experimental data obtained for the initiation of a laser discharge in a calm atmosphere and in a supersonic gas flow.
To study heat transfer during water steam condensation in the pipe, experiments were carried out with a steam supply from the top pipe edge with the gradient heatmetry. The results which allowed us ...to apply the new method for non-classical problem have shown compliance with literature data. When the pipe is inclined, the distribution of heat flux per unit area on the inner surface becomes asymmetric, that's why it is necessary to measure local heat flux in some points there. The distribution of local heat transfer coefficients on the inner surface of the pipe at different angles of inclination is obtained. The heat flux decreases in the lower part of the pipe section, which corresponds to increasing of the condensate film thickness in this area.
Anomalous heat transfer enhancement in turbulent separated air flow over a long groove of moderate depth made in a plate inclined at an angle of 45° to the freestream is revealed both experimentally ...and numerically. The region under investigation includes a rectangle heated to 100°C by saturated water vapor. The Reynolds number varied from 10
3
to 3 × 10
4
. Using the gradient heatmetry the twofold increase, as compared with the case of a flat plate, of the heat transfer coefficient on the groove bottom is established at the Reynolds number Re = 3 × 10
4
. The relative Nusselt number in different regions of the groove is determined both in the physical experiment and in the RANS calculations with the application of multiblock computational technologies and the SST model in the VP2/3 software package. The results are in good agreement in the turbulent flow regime at Re = (5, 10, and 30) × 10
3
.
The new method for study of film boiling of subcooled water is proposed. Applying the heterogeneous gradient heat flux sensors (HGHFS) allows first the experimental determination of heat flux and ...heat transfer coefficient (HTC) when subcooled water is boiling at the spherical surface preheated up to 673 K. Maximum of the heat flux was fixed when the sphere contacted with subcooled water (ΔT = 80 K). Incessant and synchronous measurement of heat flux, surface temperature, and observation of vapor film were implemented. Traditional boiling curve was also obtained. Functionality of HGHFS under the complicated conditions of experiment was confirmed.
Abstract
This paper presents experimental study of heat transfer during film condensation of saturated water steam on the outer surface of the inclined pipe by gradient heatmetry. Heat flux per unit ...area was measured by gradient heat flux sensors made of a single-crystal bismuth. The experimental results are presented in the graphs of heat flux per unit area dependence on time and azimuthal angle. The highest average heat transfer coefficient during condensation of α = 6.94 kW/(m
2
• K) was observed when the pipe was inclined at the angle of ψ = 20 °. This value exceeds one obtained on a vertical pipe by 14.9 %. Heat transfer enhancement during condensation of saturated water steam on inclined pipe is associated with changes in condensate film flow. Another part of experiments was made by simultaneously using of gradient heatmetry and condensate flow visualization. Experimental results confirmed the applicability and high informative content of proposed comprehensive method. Comprehensive study of heat transfer during condensation confirmed that heat flux per unit area pulsations may be explained by the formation of individual drops, their coalescence, and drainage from the sensor surface.
The flow and heat transfer of the finned cylinder are investigated by the methods of PIV, gradient heat flux measurement and thermometry. On the hollow model of a circular cylinder heated by ...saturated water vapor at atmospheric pressure, five annular fins of 20 mm high and of 10 mm thick are mounted. For the first model, the "acting" fin was hollow, for the second, it was solid and made of titanium alloy VT22. The remaining four fins simulated flow blockage. To visualize the flow in the intercostal space, the fin simulators were made of Perspex. Battery gradient heat flux sensors (GHFSs) with volt-watt sensitivity of 10 mV/W, were installed on the "acting" fin at different distances from the surface of the carrier cylinder. The cylinder rotated around its axis at an angle of φ = 0...180°, which made it possible to obtain the distribution of heat flux over the surface of the fin. By combining PIV diagnostics and gradient heat flux measurement, it was possible to obtain a complex 3D structure of the flow in the intercostal space and the distribution of heat flux on the surface of the fin. The values of the local heat transfer coefficients (HTCs) were also obtained. The dependence of HTC on flow mode and intercostal space is revealed. Comparison of the obtained characteristics for models with hollow and solid fins allowed us to determine in experiment fin efficiency for different intercostal spaces and free-stream velocities. The combination of all three technologies opens up new possibilities in study of flow and convective heat transfer.
The new method of study heat transfer is the gradient heat flux measurement. It allows measuring the heat flux per unit area for unsteady film boiling at the spherical surface. For the first time ...distribution over the surface of heat flux per unit area and heat transfer coefficient were derived. The time dependences for the local heat flux per unit area and heat transfer coefficient were obtained in different angles of the spherical surface. The spherical surface was made of titanium VT 22 with a diameter of 20 mm. High-speed shooting supplemented the experimental curves with a frequency of 1 kHz. The error of heat flux per unit area measurement was not exceeding 12%.
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