Two-dimensional interaction between an incident shock and a turbulent boundary layer in the presence of an entropy layer

The results of an experimental and numerical investigation of flow and heat transfer in the region of the interaction between an incident oblique shock and turbulent boundary layers on sharp and blunt plates are presented for the Mach numbers M_∞ = 5 and 6 and the Reynolds numbers Re_∞L = 27×10⁶ and 14×10⁶. The plate bluntness and the incident shock position were varied. It is shown that the maximum Stanton number St _m in the shock incidence zone decreases with increase in the plate bluntness radius r to a certain value and then varies only slightly with further increase in r. In the case of a turbulent undisturbed boundary layer heat transfer is diminished with increase in r more slowly than in the case of a laminar undisturbed flow. In the presence of an incident shock the bluntness of the leading edge of the flat plate results in a greater decrease in the Stanton number than in the absence of the shock. With increase in the bluntness of the leading edge of the plate the separation zone first sharply lengthens and then decreases in size or remains constant.     

Interaction of intersecting shocks with a flat-plate boundary layer in the presence of an entropy layer

Gas flow and heat transfer on the surfaces of sharp and blunt plates is experimentally investigated in the presence of two forward-looking wedges at the Mach numbers M = 5, 6, and 8 and the Reynolds numbers up to Re_∞L = 27×10⁶. It is shown that the entropy layer generated by a small bluntness of the leading edge of the plate can considerably change the heat transfer, the gas pressure, and the friction in the zone of interference of the shock with the plate boundary layer. Under certain conditions a small plate bluntness can also lead to a qualitative change in the flow structure. The effect of constriction of the channel between the wedges on the interference flow is studied.     

Leading edge bluntness effect on the flow in a model air-inlet

Flow in an idealized air-inlet with plane walls and a rectangular cross-section is experimentally investigated. The air-inlet is mounted on a plate, at a distance well removed from its leading edge. The experiments were conducted in a Ludwig tube at M_∞ = 5 and Re_∞L = 23×10⁶ and 13×10⁶. A panoramic (optical) technique of measuring the heat transfer coefficient is for the first time applied to study the internal flow in an air-inlet. The data on the effect of the bluntness of the leading edges of the plate and the air-inlet cowl on the heat transfer coefficient distribution and the flow structure within the air-inlet are obtained. It is shown that in an air-inlet with large channel constriction an even small bluntness of the plate or the cowl can lead to global changes in the flow structure.     

An experimental study of diabatic spiral vortex flow

In spiral vortex flow, between concentric cylinders with the inner cylinder rotating and the outer stationary, the addition of a thermal gradient across the gap is a known complicating factor. The present diabatic study for narrow and wide gaps (radius ratios N=0.955 and N=0.8), with a heated outer and adiabatic inner cylinder, was undertaken to investigate this problem. The heat transfer characteristics and the modes of transition have been investigated together with the relationship between them. Using standard on-line digital computer techniques, the onset of vortex flow and its higher transitions have been shown to cause a sharp increase in Nusselt number. At higher Taylor numbers, of the order of 10⁶, a marked change in the Nusselt number occurs with the onset of the transition to periodic turbulent vortex flow. Outer wall heating is seen to affect the modes of transition. Diabatic critical Taylor numbers are much higher than those for adiabatic conditions and are found to depend on the close approach of the vortices to the outer wall     

Laminar heat exchange on sharp and blunt plates in a hypersonic air flow

The results of an experimental investigation and numerical simulation of heat exchange are given for sharp and blunt plates in a hypersonic air flow. The experiments were carried out in a Ludwig-type wind tunnel at hypersonic Mach numbers and a Reynolds number Re_L which varied over the range from 0.24 10⁶ to 1.31 10⁶. The bluntness radius r was varied over the range from 0.008 mm (almost sharp plate) to 4 mm (the corresponding Reynolds numbers Re_r from 15 to 4 10⁴). The numerical simulation was carried out by solving the complete two-dimensional Navier-Stokes equations. The experimental data were correlated using the well-known viscous hypersonic interaction parameters.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, 2005, pp. 168–180. Original Russian Text Copyright © 2005 by Borovoi, Egorov, Skuratov and Struminskaya.     

Interaction between an Inclined Shock and Boundary and High-Entropy Layers on a Flat Plate

The flow structure and heat exchange in the zone of interference between an inclined shock and the surface of a flat plate are investigated experimentally and theoretically as functions of many parameters, the interference being studied in both the presence and the absence of bluntness of the leading edge. The experiments were carried out at Mach numbers M = 6, 8, and 10 and the Reynolds numbers Re _L , calculated using the plate length L = 120 mm and the free-stream parameters, varied over the range from 0.24 ? 10⁶ to 1.31 ? 10⁶. The bluntness radius of the leading edge of the plate, the intensity of the impinging shock, and its location with respect to the leading edge were varied. The numerical simulation was carried out by solving the complete two-dimensional Navier-Stokes equations and averaged Reynolds equations using the q-ω turbulence model. The laminar boundary layer became turbulent inside the separation zone induced by the shock. It is shown that the plate bluntness significantly reduces the heat exchange intensity in the interference zone, this effect intensifying with increase in the Mach number.     

Transient behavior of heat removal from a cylindrical heat storage vessel packed with spherical porous particles

The transient heat transfer behavior in the case of heat removal from a cylindrical heat storage vessel packed with spherical particles was investigated experimentally for various factors (flow rate, diameter of spherical particles packed, temperature difference between flowing cold air and spherical particles accumulating heat, and physical properties of spherical particles). The experiments were covered in ranges of Reynolds number based on the mean diameter of spherical particles packed Re_d = 10.3–2200, porosity?=0.310 to 0.475, ratio of spherical particle diameter to cylinder diameterd/D = 0.0075–0.177 and ratio of length of the cylinder to cylinder diameterL/D=2.5–10. It was found that especially the flow rate and the dimension of spherical particles played an important role in estimating the transient local heat transfer characteristics near the wall of the cylindrical vessel in the present heat storage system. As flow rate and diameter of spherical particles were increased under a given diameter of the cylinder heat storage vessel, the mean heat transfer coefficient between the flow cold air and the hot spherical particles increased and the time period to finish removing heat from the vessel reduced. In addition, the useful experimental correlation equations of mean heat transfer coefficient between both phases and the time period to finish removing heat from the vessel were derived with the functional relationship of Nusselt numberNu _d=f [modified Prandtl numberPr ^* (d/D), Re_d) and Fourier numberFo = f(d/D, L/D, Pr^*, Re_d).     

Local Nusselt number and temperature field in turbulent flow through a heated square-sectioned U-bend

Local heat transfer coefficients and temperature distributions within the fluid for air flow around a 180° square-sectioned bend have been measured. The ratio of bend radius to hydraulic diameter of the duct is 3.35:1 and the flow entering the bend is sensibly fully developed. Measurements of air and wall temperatures span a range of Reynolds numbers from 9.9 × 10³ to 9.2 × 10⁴ with the principal emphasis given to the case of Re ? 5.6 × 10⁴. This Reynolds number and geometric configuration coincide with that of a companion LDA study carried out by Chang et al¹ which provides detailed maps of the mean and turbulent velocity fields. The data show that by 45° into the bend the heat transfer coefficients on the inner convex wall of the bend drop markedly while those on the other walls increase. By 90° the ratio of the heat transfer coefficients at the mid positions of the concave and convex walls is more than 2:1. Nevertheless this ratio is less than would be anticipated from considering two-dimensional flow on weakly curved surfaces. There is a general consistency between the velocity and the temperatyre field data in the heated fluid     

Supersonic Laminar-Turbulent Gas Flow Past a Circular Cylinder

The flow past a circular cylinder with an isothermal surface at Mach numbers M=2 and 5 and Reynolds numbers ranging from 10⁴ to 10⁸ is investigated on the basis of the Reynolds equations using a differential two-equation turbulence model. The effect of flow turbulization on the separation point displacement, the separation zone length, the maximum velocity reduction in the separation zone, and the increase in the heat flux at the rear stagnation point is determined.     

Heat transfer in asymmetric transverse flows over bundles of tubes

Heat transfer was studied on a separate transverse row of tubes with the relative pitch ofs ₁/d=1.5 and a staggered bundle with the relative pitches ofs ₁/d×s ₂/d=1.15×0.98. A test tube in the transverse row was used with a variable displacement from the symmetric position. The tube bundle was placed at different gaps from the shell wall. Experiments were performed in air and water in the range ofRe from 10³ to 6×10⁵. Asymmetric flows over transverse rows are accompanied by augmented heat transfer rates and steady state lift force which becomes higher with the amount of displacement. The presence of the shell wall introduces alterations in the thermal and fluid dynamics over outside tubes in a bundle.     

Hypersonic interference heating in the vicinity of surface protuberances

The understanding of the behaviour of the flow around surface protuberances in hypersonic vehicles is developed and an engineering approach to predict the location and magnitude of the highest heat transfer rates in their vicinity is presented. To this end, an experimental investigation was performed in a hypersonic facility at freestream Mach numbers of 8.2 and 12.3 and Reynolds numbers ranging from Re _∞/m = 3.35 × 10⁶ to Re _∞/m = 9.35 × 10⁶. The effects of protuberance geometry, boundary layer state, freestream Reynolds number and freestream Mach numbers were assessed based on thin-film heat transfer measurements. Further understanding of the flowfield was obtained through oil-dot visualizations and high-speed schlieren videos. The local interference interaction was shown to be strongly 3-D and to be dominated by the incipient separation angle induced by the protuberance. In interactions in which the incoming boundary layer remains unseparated upstream of the protuberance, the highest heating occurs adjacent to the device. In interactions in which the incoming boundary layer is fully separated ahead of the protuberance, the highest heating generally occurs on the surface just upstream of it except for low-deflection protuberances under low Reynolds freestream flow conditions in which case the heat flux to the side is greater.     

An experimental investigation of the combined effects of surface curvature and streamwise pressure gradients both in laminar and turbulent flows

Flow and heat transfer characteristics over flat, concave and convex surfaces have been investigated in a low speed wind tunnel in the presence of adverse and favourable pressure gradients (k), for a range of –3.6 × 10^–6 ≤ k ≤ +3.6 × 10^–6. The laminar near zero pressure gradient flow, with an initial momentum thickness Reynolds number of 200, showed that concave wall boundary layer was thinner and heat transfer coefficients were almost 2 fold of flat plate values. Whereas for the same flow condition, thicker boundary layer and 35% less heat transfer coefficients of the convex wall were recorded with an earlier transition. Accelerating laminar flows caused also thinner boundary layers and an augmentation in heat transfer values by 28%, 35% and 16% for the flat, concave and convex walls at k = 3.6 × 10^–6. On the other hand decelerating laminar flows increased the boundary layer thickness and reduced Stanton numbers by 31%, 26% and 22% on the flat surface, concave and convex walls respectively. Turbulent flow measurements at k = 0, with an initial momentum thickness Reynolds number of 1100, resulted in 30% higher and 25% lower Stanton numbers on concave and convex walls, comparing to flat plate values. Moreover the accelerating turbulent flow of k = 0.6 × 10^–6 brought about 29%, 30% and 24% higher Stanton numbers for the flat, concave and convex walls and the decelerating turbulent flow of k = –0.6 × 10^–6 caused St to decrease up to 27%, 25% and 29% for the same surfaces respectively comparing to zero pressure gradient values. An empirical equation was also developed and successfully applied, for the estimation of Stanton number under the influence of pressure gradients, with an accuracy of better than 4%.     

Heat transfer from the heated concave wall of a return bend with rectangular cross section

The characteristics of the turbulent heat transfer along the heated concave walls of return bends which have rectangular cross sections with large aspect ratio have been examined for various clearances of the ducts in detail. The experiments are carried out under the condition that the concave walls are heated at constant heat flux while the convex walls are insulated. Water as the working fluid is utilized. Using three kinds of clearance of 9, 34, and 55 mm, the Reynolds number in the turbulent range are varied from 5×10³ to 8×10⁴ with the Prandtl numbers ranging from 4 to 13. As a result it is elucidated that both the mean and the local Nusselt numbers are always greater than those for the straight parallel plates or for the straight duct, respectively. This is attributed to Görtier vortices, which are visualized here. It is also found that the more the clearance increases, the more both the local and the mean Nusselt numbers increase. Correlation equations for the mean and the local Nusselt numbers are determined in the range of parameters covered. Introducing the Richardson number, it appears that the local Nusselt number,Nu _x , may be described as the following equation:Nu _x =447.745 ·Re _x ^1.497 ·De _x ^?1.596 ·F ^0.960 ·Pr ^0.412     

Study of dynamic structure and heat and mass transfer of a vertical ceramic tiles dryer using CFD simulations

In this study, we developed a two-dimensional Computational Fluid Dynamics (CFD) model to simulate dynamic structure and heat and mass transfer of a vertical ceramic tiles dryer (EVA 702). The carrier’s motion imposed the choice of a dynamic mesh based on two methods: “spring based smoothing” and “local remeshing”. The dryer airflow is considered as turbulent (Re = 1.09 × 10⁵ at the dryer inlet), therefore the Re-Normalization Group $k - \in$ model with Enhanced Wall Treatment was used as a turbulence model. The resolution of the governing equation was performed with Fluent 6.3 whose capacities do not allow the direct resolution of drying problems. Thus, a user defined scalar equation was inserted in the CFD code to model moisture content diffusion into tiles. User-defined functions were implemented to define carriers’ motion, thermo-physical properties… etc. We adopted also a “two-step” simulation method: in the first step, we follow the heat transfer coefficient evolution (H_c). In the second step, we determine the mass transfer coefficient (H_m) and the features fields of drying air and ceramic tiles. The found results in mixed convection mode (Fr = 5.39 at the dryer inlet) were used to describe dynamic and thermal fields of airflow and heat and mass transfer close to the ceramic tiles. The response of ceramic tiles to heat and mass transfer was studied based on Biot numbers. The evolutions of averages temperature and moisture content of ceramic tiles were analyzed. Lastly, comparison between experimental and numerical results showed a good agreement.     

Experimental investigation of natural convection in a vertical rib-roughened channel with asymmetric heating

An experimental study in an open-ended vertical channel is carried out in order to describe the fluid dynamics and heat transfer of transient free convection inside a vertical rib-roughened channel asymmetrically heated at various uniform heat fluxes (650, 700, and 780 W/m²) corresponding to various modified Rayleigh numbers (3.65 × 10⁶, 3.93 × 10⁶ and 4.4 × 10⁶). Two ribs are symmetrically located on each wall. The investigations focused more specifically on the influence of the ribs positions inside the channel and the modified Rayleigh number (Ra^*) both in steady-state regime and during the transitional phase occurring just after the start of the heating on the flow structure and the heat transfer performance. The results showed the appearance of large-scale flow instabilities which will develop and propagate until the development of the pocket-like vortex (reversed flow). Also, the formation and breakup of recirculation eddies, vortex banishment, besides that a separation and shifting of the boundary layer from one wall to another are identified. The best position of the ribs for heat extraction depends on the magnitude of the Rayleigh number. In that case, the top position is the optimal position for the small and the moderate modified Rayleigh numbers.     

Mixed convection and conduction heat transfer in open cavities

A numerical study has been carried out on mixed convection and conduction in open cavities. The study covers the Rayleigh number from 10 to 10⁶, the Reynolds number from 1 to 10³ andk _r =k _w /k _a from 1 to 100 forPr=0.72 (air) and cavity aspect ratioB from 0.8 to 1.3. The flow is assumed to be laminar and two-dimensional. The density variation is taken into account by the Boussinesq approximation. The controlvolume approach is used for solving the governing equations of conjugate heat transfer involving conduction in the walls. Streamlines and isotherms in the system are produced and the results are represented in terms of the Nusselt number as function of other parameters. Correlations are derived to calculate heat transfer through the cavity openings.     

Intensity of convective mass/heat transfer in a rotary regenerator rotor with the transverse needle-fins

A forced convective mass transfer coefficient was electrochemically measured for a cylindrical bundle of transverse needle-fins ?1 × 10.9, applied as the rotor porous matrix of a rotary heat regenerator. The baffle inside the rotor was present. The technique based on the ferricyanide–ferrocyanide redox reaction controlled at the cathode, in the presence of a sodium hydroxide based electrolyte, was used in this experiment. A set of the six neighbouring fins, connected in parallel, was the cathode. The distribution of the mass transfer coefficient according to different static rotor angle position and the mean mass transfer Chilton–Colburn coefficient correlation j _M  = j _M (Re) for rotation numbers, Ro: 0, 0.8, 1.6 and 2.0 were stated in the mean Reynolds number, Re, range 180–985. The comparison was made between the convective heat fluxes of the pin-fins and the sheet rotor, for Ro = 0.     

Study of the aerodynamic stability of a blunt cone with a side flap in a hypersonic flow

The stationary and time-dependent aerodynamic coefficients of a slender blunt cone with a flap located near the base section of the model are experimentally investigated. The freestream parameters (M_∞ = 6, Re _L = 0.88 × 10⁷, and γ = 1.4) ensured a turbulent regime of flow over the conical surface and the flap. At high angles of attack (α ～ 10°) laminar-turbulent transition is observable in the separation zone on the leeward side of the body. Emphasis is placed on the determination of the trimming angles of attack for different positions of the center of rotation and the static and dynamic stability coefficients (the model oscillation damping coefficient).     

Effect of rib angle orientation on local mass transfer distribution around sharp 180 deg turn with rib-turbulators mounted in entire two-pass channels

The local mass transfer distributions around sharp 180 deg turn with rib-turbulators in a relatively short (L/D=4) two-pass, square channel were determined via the naphthalene sublimation technique. The rib height-to-hydraulic diameter ratio (e/D) was 0.05 and the rib pitch-to-height ratio (p/e) was 10. Experiments were conducted for five attack angles (α=90,45,60,?45, and ?60 deg), and for three Reynolds numbers (3.0×10⁴, 6.0×10⁴ and 9.0×10⁴). Results show that the rib-roughened wall Sherwood numbers after the turn are higher than those in the turn region, which, in turn, are higher than those before the turn due to the ribs installed in the turn region. While in the previous studies, the ratio Sh/Sh ₀ in the turn region was found to be lower than that in the before turn region, because of the absence of rib in the turn region. The rib angle and rib orientation have significant effect on both the local and average Sherwood number ratios. The average Sherwood number ratios (Sh _m /Sh ₀) for α = 60 deg have the highest values, then comes the case of α = 45 deg, and the Sherwood number ratios (Sh _m /Sh ₀) for α = 90 and ?45 deg are the lowest. Correlations for the average rib-roughened wall Sherwood number ratios for the before-turn, in-turn and after-turn segments are provided.     

Influence of the aspect ratio on boiling flows in rectangular mini-channels

This article presents experiments conducted with two single rectangular mini-channels of same hydraulic diameter (1.4 mm) and different aspect ratios for conditions of horizontal boiling flow. The Forane® 365 HX used was subcooled (ΔT_sub = 15 °C) for all the boiling curves presented in the paper. Local heat transfer coefficients were measured for heat flux ranging from 25 to 62 kW m⁻² and mass flux from 200 kg m⁻² s⁻¹ to 400 kg m⁻² s⁻¹. The boiling flows were observed with two different cameras (depending on the flow velocity) through a visualization window. The flow patterns in the two channels were compared for similar conditions. The results show that the boiling heat transfer coefficient and the pressure drop values are different for the two single mini-channels. For low heat flux condition, the channel with lowest aspect ratio (H/W = 0.143) has a higher heat transfer coefficient. On the other hand, for high heat flux condition, the opposite situation occurs, namely the heat transfer coefficient becomes higher for the channel with highest aspect ratio (H/W = 0.43). This is probably due to the earlier onset of dryout in the channel with lowest aspect ratio. For the two cases of heating, the pressure drop for the two-phase flow remains lower for the channel with lowest aspect ratio. These results show that the aspect ratio plays a substantial role for boiling flows in rectangular channels. As for single-phase flows, the heat transfer characteristics are significantly influenced (even though the hydraulic diameter remains the same) by this parameter.