A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

柱塞泵油膜压力分析的高效有限单元法

The paper concerns numerical analysis of pressure distribution of an oil film on the valve plate in the variable height gap of an axial piston pump. The analysis employs the finite element method. For determination of oil pressure variations in the gap, the Reynolds equation, commonly applied in the theory of lubrication, is applied. The equation is solved numerically with the use of self-developed program based on the finite element method. In order to obtain high accuracy of the results, an adaptive mesh refinement based on residual estimations of solution errors is applied. The calculation results are represented as dependent on the geometric and working parameters of the pump.     

Numerical investigation of unsteady mixing mechanism in plate film cooling

A large-scale large eddy simulation in high performance personal computer clusters is carried out to present unsteady mixing mechanism of film cooling and the development of films.Simulation cases include a single-hole plate with the inclined angle of 30° and blowing ratio of 0.5,and a single-row plate with hole-spacing of 1.5D and 2D(diameters of the hole).According to the massive simulation results,some new unsteady phenomena of gas films are found.The vortex system is changed in different position with the development of film cooling with the time marching the process of a single-row plate film cooling.Due to the mutual interference effects including mutual exclusion,a certain periodic sloshing and mutual fusion,and the structures of a variety of vortices change between parallel gas films.Macroscopic flow structures and heat transfer behaviors are obtained based on 20 million grids and Reynolds number of 28600.     

The influence of heat transfer coefficient on cooling time in injection molding

The cooling time of the polymer melt in injection molding is a major part of the total time of the processing cycle. Estimation of cooling time is therefore important. The effect of heat transfer coefficient, thermal conductivity and diffusivity on the cooling time is studied theoretically, showing high effects of variations in heat conductivity on cooling time.     

A parametric investigation of vane pressure side cutback film cooling by dual luminophor PSP

Because of its geometry, the vane trailing edge is one of the most difficult regions to be cooled. A trailing edge cutback cooling system is one of the most effective solution for cooling the trailing edge of high-pressure gas turbine nozzle vanes. In this study, a parametric analysis of the thermal performance of a nozzle vane cascade with a pressure side cooling system including two rows of cylindrical holes and a trailing edge cutback featuring 8 rectangular slots was carried out by using dual luminophor (binary) PSP technique. Coolant to mainstream mass flow rate (MFR), density ratio (DR), main flow Mach number (Ma_2is) and turbulence intensity level (Tu₁) and the state of the approaching boundary layer were the considered parameters. Binary PSP was able to measure the coolant concentration independently from temperature, thus allowing to compute the true adiabatic film cooling effectiveness in a complex environment. MFR was shown to have a strong impact on both holes and cutback performance. The thermal protection over the cutback region was promoted by high Ma_2is and high DR values, while Tu₁ and the boundary layer state only marginally affected the thermal behavior, especially at high MFR.     

Similarity analysis of model experiments for film cooling in gas turbines

This report investigates our present ability to predict the thermal performance of film cooling arrangements used to protect the hot components of gas turbines. The required information is usually obtained by model experiments carried out at near room temperature as opposed to the high temperature encountered in the gas turbines. Dimensional or similarity analysis is used to develope the functional relationships for film effectiveness and convective heat transfer and the use of mass transfer experiments with foreign gas injection and naphthalene sublimation based on the heat-mass transfer analogy is discussed. The law of superposition is used to describe the combined effects of film cooling, surface convection or radiation and frictional heating. An order of magnitude estimate indicates to what extent local temperature gradients are alleviated in the cooled walls by internal heat conduction.     

A contribution to the Kryloff-Bogoliuboff theory in nonlinear mechanics

Summary An extension of the Kryloff-Bogoliuboff method in the theory of nonlinear oscillation has been developed.The method is intended primarily for the resonant analysis of systems exhibiting transient as well as steady motion, and describes subharmonic and superharmonic responses in complete generality.The method is applied to the equation describing the motion of a harmonically driven nonlinear oscillator. Higher approximation is obtained and the stability of the solution has been examined. The results of this method, when compared with exact solutions, show that this technique affords a more workable means for studying the responses of the nonlinear system harmonic forces.

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Übersicht Es wird eine Erweiterung für die von Kryloff und Bogoliuboff angegebene Methode der Theorie nichtlinearer Schwingungen beschrieben. Das Verfahren ist vorwiegend für die Resonanzanalyse bei stationären oder instationären Bewegungen geeignet. Subharmonische und superharmonische Bewegungen lassen sich in voller Allgemeinheit damit berechnen.Das Verfahren wird am Beispiel einer Gleichung demonstriert, die die Bewegungen eines harmonisch erregten nichtlinearen Schwingers beschreibt. Höhere Näherungen werden abgeleitet, und die Stabilität der Lösungen wird untersucht. Ein Vergleich mit exakten Lösungen zeigt, daß das angegebene Verfahren gut brauchbar für die Berechnung der Bewegungen nichtlinearer Systeme mit harmonischen Erregungen ist.

     

A broad reconsideration of anti-vortex film cooling method using numerical optimization and an improved heat-flux model

This paper represents the detailed results of an evolutionary optimization framework towards the exploration of vortex mechanisms leading to effective anti-vortex film cooling. In this regards, several arrangements of triple cooling holes were studied on flat and curved geometries using differential-evolution optimization algorithm and a modified Reynolds-stress based flow solver. Depending on the flow and geometric parameters, four distinct types of vortex interaction with different cooling mechanisms were identified. The vortex-trapping mechanism, observed in the optimized upstream arrangement acts through imposing a mild downwash over the main counter-rotating vortex pair and provides the best cooling effectiveness for the low injection angle (less than 30°) cases. The vortex-suppression and -balancing are the optimal possible solutions of the adjacent arrangement. The latter is the classic well-known type of anti-vortex cooling, while the former provides a sudden strong controlling potential for high blowing ratios (higher than 1.0) and high injection angle film cooling. For the non-flat surfaces the triple holes effectively perform up to blowing-ratio of 2.0. However, the reverse-vortex-trapping mechanism occurring in the downstream arrangement is recommended for convex surfaces, while the adjacent arrangement is the choice for concave regions. In general, there is a possibility of reducing the coolant consumption about 30% through increasing the pitch-to-diameter ratio, while the values of cooling-effectiveness still remain in an acceptable range.     

A contribution to the theory of streamer breakdown

The development of a systematic theory of streamer breakdown of a gas requires the consideration of the transport of the region of ionization toward the ionized gas in an electric field depending on the form of the streamer, which in turn is determined by the transport mechanisms [1–3]. In this form the problem is very complicated,and the theory takes the path of investigation of different qualitative models of a streamer [4]. It is assumed in [4] that the rates of anode-directed and cathode-directed streamers are determined by the drift velocity of the electrons. The mechanism of propagation of anode-directed streamers is taken to be the development of avalanche from the leading front of the electrons traveling to the anode. On the side of the cathode, electrons before the front of the cathodedirected streamer are produced due to the transport of radiation from the ionized region [1]. It is shown in [5] that direct photo-ionization is ineffective because of the small range of the quantas, and a mechanism of development of cathode-directed streamer related to the associative ionization of excited atoms is proposed. These atoms are formed by long-span resonance photons from the wings of the spectral line. An interesting prediction of the theory [4] was a linear dependence of the velocity of the streamers on their length. This dependence was confirmed in experiments on the study of streamer breakdown initiated at the center of the discharge gap in spark chambers [6, 7]. At the same time, for streamers developing from avalanche initiated at one of the electrodes the velocity of propagation of the breakdown wave remains constant with a good accuracy in gaps having lengths of the order of 1 m. In the present work a qualitative theory is developed which permits one to calculate the velocity of the an ode-directed streamer in the case where it is independent of the length. Since for pressures of the order of atmospheric pressure the diffusion coefficient of excited atoms [8] is comparable with the electron diffusion coefficient, the effect of radiation transport is disregarded. The stability of the front of the streamer to infinitely small perturbations is investigated. It is shown that, when the finite thickness of the front is taken into consideration, the streamer is stable. It is unstable in the approximation of infinitely thin leading fronts.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 56–65, January–February, 1973.The authors thank A. A. Vedenov, E. P. Velikhov, A. P. Napartovich, and O. B. Firsov for valuable discussions.     

A contribution to the great Riemann solver debate

The aims of this paper are threefold: to increase the level of awareness within the shock-capturing community of the fact that many Godunov-type methods contain subtle flaws that can cause spurious solutions to be computed; to identify one mechanism that might thwart attempts to produce very-high-resolution simulations; and to proffer a simple strategy for overcoming the specific failings of individual Riemann solvers.     

A contribution to the stability of circular plates

Archive of Applied Mechanics - The dynamical response of thin circular plates and their stability, resulting from the action of an arbitrary load which is a spacetime function, has been examined....     

A numerical investigation of new film cooling hole configuration at the leading edge of asymmetrical turbine blade: part A

The focus of the first part of this numerical study is to investigate the effects of two new configurations: (1) slot with cylindrical end and (2) slot with median cylindrical hole, generated by the combination between two film cooling configurations: cylindrical hole and uniform slot. Computational results are presented for a row of coolant injection holes on each side of an asymmetrical turbine blade model near the leading edge. For each configuration, three values of the radius are taken: R = 0.4, R = 0.8 and R = 1.2. The six cases simulations, thus obtained, are conducted for the same density ratio of 1.0 and the same inlet plenum pressure. A new parameter, Rc, is defined to measure the rate of blade coverage by the film cooling. Results show that, at the pressure side; for the two new configurations, the six studied cases exceed the case baseline in cooling effectiveness term with the best result obtained for R = 0.8 (case 2). For the suction side, only configurations with R = 0.4 (cases 1 and 4) provide an increase of film effectiveness compared to the case baseline. The following configuration: Cases 1 or 4 at the suction side and case 2 at the pressure side, gets the best thermal protection because of their higher coverage and strong cooling effectiveness.     

Laser measurement techniques applied to turbulent combustion in piston engines

The application of experimental laser diagnostic techniques to the study of turbulent flame propagation in homogeneous-charge internal combustion engines is demonstrated. A variety of laser refraction and scattering techniques are employed: the qualitative shape and detailed internal turbulence structure of the flame surface is investigated using laser schlieren photography; the flame location and local flame speed are measured using multibeam laser refraction; the instantaneous flame thickness is measured using Rayleigh scattering; gas temperatures are measured using Rayleigh and Raman scattering; gas velocity, turbulence intensities, and time scales of turbulence are measured using laser Doppler velocimetry. The problem of cyclic variation bias in turbulence measurements is addressed using conditional sampling procedures that involve the simultaneous implementation of multiple diagnostics. This paper focuses on the thermodynamic and fluid mechanic properties of interest to turbulence combustion studies, illustrating how laser diagnostics can be used to research the critical areas of interest.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 and printed in AMD, Vol. 66     

A numerical evaluation on the effect of sister holes on film cooling effectiveness and the surrounding flow field

A numerical study was performed to evaluate the effectiveness of the novel sister hole film cooling technique. Two secondary coolant holes bound the primary coolant hole slightly downstream of its midpoint, intended to minimize the primary vortex pair and improve cooling performance. An unstructured hexahedral mesh was generated and the realizable k–ε turbulence model with near-wall modeling was used in these simulations. Blowing ratios of 0.2, 0.5, 1.0, and 1.5 were simulated to evaluate the applicability of sister holes in practical applications. It was found that sister holes significantly improved cooling performance over the entire computational domain, particularly at high blowing ratios. These results arose by countering the primary vortex pair with a secondary pair from these sister holes, ultimately maintaining flow adhesion where the coolant stream would have otherwise separated.