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[en] Two-dimensional hydromagnetic flow and heat transfer of Walters-B fluid towards a stagnation point region over a stretching cylinder is discussed. Constitutive equations are transformed into dimensionless form by means of suitable similarity transformations. Spectral quasi-linearization method is employed to obtain the solution of similarity equations. Comparison of computed results with existing results in the limiting case of a flat sheet is also provided. Analysis of obtained results is performed through graphs to discuss the influence of emerging parameters on the velocity and temperature profiles. The flow and heat transfer characteristics are analyzed through parameters representing curvature of cylinder, velocity ratio parameter, magnetic parameter, and Weissenberg number. The curvature of the cylinder has significant impact on the velocity and temperature. A magnetic field applied externally suppresses the bulk motion and alters the momentum boundary layer thickness. The drag and heat transfer rate on the surface of the cylinder are examined through skin friction and heat transfer coefficients. Furthermore, streamlines are drawn to see the flow pattern. (author)
[en] Performance of PFP system is verified by the properly performed experiments so engineers usually apply PFP based on its performance test results. However, in some cases, PFP cannot be applied as its initial design due to the constraints of space, workability, cost and other complicated reasons. In cases of low utilization against in-place load and very limited fire exposure, partially or thinly applied PFP could be acceptable if response under fire is fully assessed to ensure the integrity of the structure. To perform the fire redundancy analysis, temperature field of structural members should be determined at first and finite element tools are utilized for this purpose. In this paper, FAHTS (Heat transfer analysis module of USFOS) and LS-dyna are adopted for simulations. Considering the characteristics and functional limitations of each tool, heat transfer analysis with and without PFP are carried out. By comparing the results with analytical solution, proper modeling techniques to simulate the heat transfer analysis for each software are confirmed. In this paper, case studies for heat transfer analysis with and without PFP are performed using commercial FE tools and characteristics are compared.
[en] This paper presents an alternative to cubic spline regularization and its weighted form applied in solving inverse thermal problems. The inverse heat transfer problems are classified as ill-posed, that is, the solution may become unstable, mainly because they are sensitive to random errors deriving from the input data, necessitating a regularization method to soften these effects. The smoothing technique proposed by cubic spline regularization ensures that the global data tend to be more stable, with fewer data oscillations and dependent on a single arbitrary parameter input. It also shows that the weighted cubic spline is able to enhance filter action. The methods have been implemented in order for the search engine to optimize the choice of parameters and weight and, thus, the smoothing gains more flexibility and accuracy. The simulated and experimental tests confirm that the techniques are effective in reducing the amplified noise by inverse thermal problem presented.
[en] Floating LNG bunkering offshore terminal (FLBT) has been adopted as a floating unit by the marine industry equipped with the natural gas liquefaction plant on the ship's deck with the practical interest in controlling the plume emission discharged from the ship. The downwash of emitted plumes has adverse consequences on the ship’s engine intake and ventilation system followed by the interference of the smoke with helicopter operations. Owing to this fact, understanding of the plume behavior is considered to be so significant in the aspect of ship design and thereby, flow visualization techniques assist to study the plume path ensuring the safety of the personnel and functioning system. The unignited flare gas emitted from the tower invites heat transfer due to the temperature difference between the atmospheric wind and the ship’s exhaust, which follows the examination of mixing enhancement of fluid mediums as well as the mitigation of plume concentration depending upon wind speeds. The parametric study is intended to investigate the plume dispersion pattern around a ship based on the flare motion and bending angle for light and strong wind speeds. It is observed that the plume gas rises higher and disperses over the larger area with wider streamline separation due to the effect of buoyancy for light wind speeds. On the other hand, the motion of flare gas is found to be narrow for strong winds restricting vertical movements due to dominant inertial forces than gravity pull. Thus, the current numerical investigation facilitates in understanding the configuration and plume distribution by the variation in streamline behavior with contours plots. In this work, the calculated results are analyzed systematically in pretty realistic conditions and simple measures are obtained, which will be applied to the preliminary design of plume stack depending on the ship’s deck arrangement.
[en] Highlights: • Optimal controlled variable of counter flow heat exchanger is constructed. • T-Q diagram analysis inducing entropy angle and thermal capacity angle is proposed. • Effect of irreversibility on terminal temperature difference is identified. • Relation between effectiveness, transfer and remanent irreversibility is revealed. - Abstract: To optimize the controlled variable of counter flow heat exchanger, T-Q diagram inducing entropy angle and thermal capacity angle is used to analyze heat exchange process. The results show that selecting stream outlet temperature as controlled variable is incapable of perceiving overall variation of thermal capacity flow rates. The change of heat exchanger effectiveness isn’t completely consistent with heat transfer irreversibility, and cannot reflect the effect of remanent (flow-imbalance) irreversibility. The terminal temperature difference imposed by heat transfer irreversibility is the same at both ends. However, the remanent irreversibility makes the terminal temperature difference of one end deviate from the other. Based on maximizing the heat exchange amount and minimizing the irreversible loss, a new controlled variable named as heat exchanger comprehensive effectiveness is constructed, which is easy to be measured and calculated. It can reflect the effect of heat exchanger effectiveness, remanent irreversibility, and heat transfer irreversibility simultaneously.
[en] Highlights: • Three parameters are used to evaluate the startup performance. • The startup is faster and overshoot is larger when the distribution is more uneven. • Heating on one evaporator with same Q with the other makes transition time longer. • Heating on one evaporator with same Q with the other makes the overshoot smaller. • Transition time is about twice as much as peak time when peak time exists. - Abstract: Loop thermosyphon with multiple evaporators is a promising device in multi-source heat transfer. The startup performance is very important for its thermal control ability. In this paper, the effect of heating power distribution on the startup of a loop thermosyphon with dual evaporators is investigated experimentally. The startup time and stationarity under different power distributions are analyzed utilizing three parameters: peak time, transition time and temperature (pressure) overshoot. The results show that the startup process is faster and the overshoot of pressure and temperature is larger when the distribution is more uneven; Heating on one evaporator with the same heating power with the other evaporator makes the startup process longer while it makes the overshoot smaller or even disappear; The transition time is approximately twice as much as the peak time when the peak time exists.
[en] We report a method for quantifying scanning thermal microscopy (SThM) probe–sample thermal interactions in air using a novel temperature calibration device. This new device has been designed, fabricated and characterised using SThM to provide an accurate and spatially variable temperature distribution that can be used as a temperature reference due to its unique design. The device was characterised by means of a microfabricated SThM probe operating in passive mode. This data was interpreted using a heat transfer model, built to describe the thermal interactions during a SThM thermal scan. This permitted the thermal contact resistance between the SThM tip and the device to be determined as 8.33 × 105 K W−1. It also permitted the probe–sample contact radius to be clarified as being the same size as the probe’s tip radius of curvature. Finally, the data were used in the construction of a lumped-system steady state model for the SThM probe and its potential applications were addressed. (paper)
[en] Post-dryout heat transfer experiments conducted on six different test facilities have been simulated with TRACE (V5.0P4). The focus of the experimental condition has been on a system pressure P=70 bars (although not exclusively) and a broad range of mass fluxes down to boiloff conditions (G=5235 kg/m2s to G=2 kg/m2s), while utilizing primary data (as opposed to reduced variables). The test sections included single-tube as well as (5x5), (6x6), and (8x8) rod-bundle geometries. Aside from a series of transient tests, all experiments were conducted under stabilized post-dryout conditions, thus providing an effective assessment of the wall-to-coolant heat transfer code performance. Also, comparisons with concomitantly measured axial void profiles were included in some cases of boiloff tests. The calculated results indicated significant disparity of the code predictive capability, which would render the development of a faithfully representative error density function challenging, as in the methodology of best-estimate calculations plus uncertainty analysis (BEPU). (author)
[en] Water at SuperCritical Pressures (SCPs) is used in modern advanced coal-fired power plants around the world that are able to reach thermal efficiencies up to 55%, operating at 25-38 MPa pressures and 540-625oC turbine inlet temperatures. However, the current nuclear plants are not able to obtain such high thermal efficiencies (typically, 30-36%), due to the conventional Rankine cycles operating at conditions that are significantly below the supercritical ones, eventually, using only saturated primary steam within the pressure range of 4.7-7 MPa. The main focus of the paper is an investigation on specifics of Deteriorated Heat Transfer (DHT) in SuperCritical Water (SCW) (liquid-like state) flowing upward in a short bare tube, using data from I. Sikorsky Kiev Polytechnic Institute (KPI). Analysis of the experimental data showed an existence of three HT regimes: 1) Normal HT; 2) Improved HT; and 3) Deteriorated HT. Starting and developing of the deteriorated HT regime is also discussed. The experimental data along with preliminary calculations were provided by KPI while the analysis and report were carried out by the authors from UOIT. (author)
[en] Highlights: • Effect of different vortex generators on heat exchanger performance is examined. • The analysis is done using finite volume method. • Six different vortex generators have been investigated. • Simple rectangular vortex generator increases heat transfer more than other. - Abstract: In this work, the effect of different vortex generators on fin-plate heat exchanger performance with a triangular channel cross-section is examined. The analysis is done using finite volume method. The effects of vortex generators in a channel are investigated by consideration of channel temperature and heat transfer coefficient. Six different vortex generators including a simple rectangular vortex generator (SRW), rectangular trapezius vortex generator (RTW), angular rectangular vortex generator (ARW), Wishbone vortex generators (WW), intended vortex generator (IVG) and wavy vortex generator (WVG) have been investigated. The observations suggest that simple rectangular vortex generator increases the heat transfer of fin-plate heat exchanger more than other models. This vortex generator increases heat transfer in the heat exchanger by 7%. However, vortex generators increase the pressure drop in heat exchanger. In addition, by increasing the height of the vortex generators the heat transfer rate is increased and the best angle of attack for the installation of vortex generator is 45°.