Heat transfer characteristics of an array of impinging pre-mixed slot flame jets

An experimental study was carried out to investigate the shape and the heat transfer characteristics of an array of three laminar pre-mixed butane/air slot flame jets impinging upwards normally on a horizontal water-cooled flat plate. The effects of jet-to-jet spacing and nozzle-to-plate distance were examined at the Reynolds number (Re) of 1000 and the equivalence ratio (?) of unity. Comparisons of the heat transfer characteristics between single and multiple slot flame jets, as well as multiple slot and round jets, were made. The between-jet interference decreased with increasing jet-to-jet spacing (s/d_e) and nozzle-to-plate distance (H/d_e). Strong interference was obtained at s/d_e = 1 and H/d_e = 2, at which the central jet was suppressed while the side jets were deflected towards their free sides. In addition, there was no minimum heat flux found in the inter-jet interacting zone, instead, a peak heat flux was obtained. Thermal performance was reduced when H/d_e became smaller than the length of the conical luminous reaction zone of the flame. A maximum average heat flux occurred at the moderate jet-to-jet spacing of s/d_e = 2.5 at Re = 1000, ? = 1 and H/d_e = 2. The resultant heat flux distribution of the central jet of a multiple slot jets system was higher than that of a single slot jet when the jet-to-jet spacing was small, but this advantage in thermal performance diminished when the jet-to-jet spacing was increased. Besides, the area-averaged heat flux of the multiple slot flame jets was higher than that of the multiple round flame jets arranged at the same geometric configuration.     

Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets

Experimental studies were carried out to investigate the flame shape and the heat transfer and wall pressure characteristics of a pair of laminar premixed butane/air flame jets impinging vertically upon a horizontal water-cooled flat plate at jet Reynolds numbers of 800, 1000 and 1200, respectively. Equivalence ratio of the butane/air mixture was maintained constantly at unity. The flame shape, the pressure distribution on the impingement plate and the heat transfer from the flame to the plate were greatly influenced by the interference occurred between the two flame jets. This interference caused a sharp pressure peak at the between-jet midpoint and the positive pressures at the between-jet area, which led to the separation of the wall jet from the impingement plate after collision. Such interference became more significant when the non-dimensional jet-to-jet spacing (S/d) and the nozzle-to-plate distance (H/d) were reduced. Heat transfer in the interaction zone between the jets was at the lowest rate due to this interference at the smallest S/d ratio of 2.6, resulting from the separation of the high-temperature inner reaction zone of the flame from the impingement plate. On the other hand, the interference enhanced the heat transfer in the interaction zone between the jets when the S/d ratio was greater than 5, by enhancing the heat transfer coefficient. The average heat flux of the impingement plate was found to increase significantly with the increasing H/d ratio until H/d=6. The present study provided detailed information on flame shape and the heat transfer and wall pressure characteristics of a twin laminar pre-mixed impinging circular flame jets, which has rarely been reported in previous studies.     

Impinging premixed butane/air circular laminar flame jet--influence of impingement plate on heat transfer characteristics

Experimental studies were performed to study the heat transfer characteristics of an impingement flame jet system consisting of a premixed butane/air circular flame jet impinging vertically upward upon a horizontal rectangular plate at laminar flow condition. There were two impingement plates manufactured with brass and stainless steel respectively used in the present study. The integrated effects of Reynolds number and equivalence ratio of the air/fuel jet, and distance between the nozzle and the plate (i.e. nozzle-to-plate distance) on heat transfer characteristics of the flame jet system had been investigated. The influence in using impingement plate with different thermal conductivities, surface emissivities and roughnesses on heat flux received by the plate was examined via comparison, which had not been reported in previous literatures. A higher resistance to heat transfer had been encountered when the stainless steel impingement plate of lower thermal conductivity was used, which led to a significantly lower heat flux at the stagnation region. However, the heat flux distribution in the wall-jet region of the plate was only slightly affected by using different impingement plates. Because of the significantly lower heat transfer, more fuel was not required to consume and existed at the stagnation region of the stainless steel impingement plate, which would be burned latter in the wall-jet region to release its chemical energy and enhance the local heat flux there.     

Heat transfer from an impinging premixed butane/air slot flame jet

Experiments were performed to study the heat transfer characteristics of a premixed butane/air slot flame jet impinging normally on a horizontal rectangular plate. The effects of Reynolds number and the nozzle-to-plate distance on heat transfer were examined. The Reynolds number varied from 800 to 1700, while the nozzle-to-plate distance ranged from 2d_e to 12d_e. Comparisons were made between the heat transfer characteristics of slot jets and circular jets under the same experimental conditions. It was found that the slot flame jet produces more uniform heat flux profile and larger averaged heat fluxes than the circular flame jet.     

Heat transfer characteristics of three interacting methane/air flame jets impinging on a flat surface

An experimental study has been conducted for three interacting methane/air flame jets (arranged in a triangular configuration) impinging normally on a flat surface. Surface heat flux distributions have been determined for various dimensionless inter-jet spacings (S/d = 3, 4, 6 and 7.58) and separation distances between the exit plane of the burners and the target plate (H/d = 2, 2.6, 5 and 7). All experiments were conducted for stoichiometric mixture at a Reynolds number of 800. The surface heat flux distributions were intimately related to flame shapes. For small inter-jet spacings and small separation distances, flames were deflected outward from the centroid of the triangular arrangement due to strong interaction between the jets. The heating was quite non-uniform at very large inter-jet spacings. Zones of low heat flux were obtained when the tip of inner reaction zones were intercepted by the plate (H/d = 2). There were sharp peaks in the heat flux distribution when the tips of the inner reaction zones just touched the impingement surface (H/d = 2.6). Heat flux distribution was non-uniform at small separation distances (H/d = 2 and 2.6). For the system of flame jets under consideration, the optimum configuration, considering the magnitude of the average heat flux and the uniformity in the heat flux distribution, was corresponding to H/d = 5 and S/d = 3.     

旋进射流冲击平板的传热实验研究

对旋进射流冲击平板时的传热进行了实验研究。通过在圆筒套管内设置一块孔板构成旋进射流喷嘴,得到了持续稳定的旋进射流。对旋进射流的流动特性作了研究,给出了旋进射流的频率与尺寸、Re的关系。用两种不同孔径的旋进射流冲击一块加热平板,并与普通的射流冲击传热作对比。结果表明,由于旋进射流与流体混合作用加剧而大大地降低了流速,使得强化传热的效果减弱,这种趋势在驻点附近尤为明显。     

Mist/steam cooling by a row of impinging jets

Mist/steam cooling has been studied to augment internal steam-only cooling for advanced turbine systems. Water droplets generally less than 10 μm are added to 1.3 bar steam and injected through a row of four round jets onto a heated surface. The Reynolds number is varied from 7500 to 22,500 and the heat flux varied from 3.3 to 13.4 kW/m². The mist enhances the heat transfer along the stagnation line and downstream wanes in about 3 jet diameters. The heat transfer coefficient improves by 50-700% at the stagnation line for mist concentrations 0.75-3.5% by weight. Off-axis maximum cooling occurs in most of the mist/steam flow but not in the steam-only flow. CFD simulation indicates that this off-axis cooling peak is caused by droplets’ interaction with the target walls.     

A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Heat transfer from a plate impinging swirl jet

Heat transfer and flow visualization experiments were conducted to investigate the performance of swirling and multi‐channel impinging jets and compare the results with those of a multi‐channel impinging jet (MCIJ) and conventional impinging jet (CIJ) for the present work at the same conditions. Swirling impinging jets (SIJs) employed the fixed blade lengths of 12.3 mm with four blades at the exit of the housing tube to divert the air flow through four narrow channels with a desired swirl angle (θ of 22.5, 41 and 50°). The MCIJ jet had the same dimensions as the SIJs, except that the narrow channels in the solid insert were vertical (θ=0°). The local and surface average Nusselt numbers of MCIJ were generally higher than those of the CIJ and SIJs. The SIJs, however, demonstrated significant improvement in radial uniformity of heat transfer compared to the MCIJ and CIJ. In the region of 2.7?X/D?0 for H/D=8 and Re=20 000, the average Nusselt number for the MCIJ was 11, 33, 72 and 98 per cent higher than that of the CIJ, θ=22.5, θ=41 and θ=50°, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Heat transfer enhancement by multiple swirling impinging jets with twisted-tape swirl generators

The flow and heat transfer characteristics of multiple swirling impinging jets (M-SIJs) with 3 × 3 in-line arrangement, on impinged surfaces are reported. The experiments were conducted with four different jet-to-jet distances (S/D = 2, 4, 6 and 8) at the constant nozzle-to-plate distance of L/D = 4. The swirling jets with the swirl numbers of 0.4 were associated with twisted tapes. The multiple conventional impinging jets (M-CIJs) were also tested, for comparison. The flow patterns on an impinged surface were visualized using oil film technique while the distributions of temperature field and Nusselt number on impinged surface were evaluated via a thermochromic liquid crystal (TLC) sheet coupled with image processing technique. The experimental results showed that the M-SIJs offered higher heat transfer rate on impinged surfaces than the M-CIJs of all jet-to-jet distances (S/D).     

Heat transfer of impinging air and liquid nitrogen mist jet onto superheated flat surface

This experimental study performs the detailed heat transfer measurements of an impinging air-liquid nitrogen mist jet onto a superheated flat surface at atmospheric pressure with reference to the design of an instant freezing facility. A selection of experimental results illustrates the interacting effects of jet Reynolds number, mass flow ratio of air to liquid nitrogen flows and separation distance on the spatial distributions of heat transfer over the impinging surface. Mechanism associated with phase change of impacting droplets generates an enhanced and uniformly distributed heat transfer region centered on the stagnation point. A narrow oval-ring region encapsulating the enhanced core transits heat transfer from the wetting regime of complete evaporation to the non-wetting rebound regime. Stagnation heat transfer augmentation factor in the range of 1.2-2.8 times of the air-jet level is achieved. An empirical correlation based on the experimental data, which is physically consistent, has been developed to permit the evaluation of stagnation heat transfer.     

Numerical simulation of flow and heat transfer from slot jets impinging on a cylindrical convex surface

Flow and heat transfer characteristics of slot jets impingement to a cylindrical convex surface are numerically investigated.Suitable turbulence models have been determined through comparison with the experimental data.Flow structures are described and impingement heat transfer characteristics are discussed.The effects of Re,H/B and D/B on single-slot jets impingement heat transfer are analyzed and heat transfer characteristics of multiple-slot jets are investigated.The results show that:Gas flows along the convex surface and boundary layer separation occurs in both single and multiple-slot jets impingement.A maximum stagnation Nu appears at H/B=8 and the local Nu decreases with increasing H/B in the region far away from the stagnation.The Nu in the stagnation region decreases with increasing D/B but the Nu is nearly the same in the region far away from the stagnation.Pressure gradient is an important factor on heat transfer enhancement.Correlations of the Num for single-slot,double-slot and quadric-slot jets impinging on a convex surface are obtained.It indicates the effects of Re and D/B on Num could become more important in less slot jets impingement.     

A numerical study of laminar flame speed of stratified syngas/air flames

Numerical simulations are performed to study the flame propagation of laminar stratified syngas/air flames with the San Diego mechanism. Effects of fuel stratification, CO/H₂ mole ratio and temperature stratification on flame propagation are investigated through comparing the distribution of flame temperature, heat release rate and radical concentration of stratified flame with corresponding homogeneous flame. For stratified flames with fuel rich-to-lean and temperature high-to-low, the flame speeds are faster than homogeneous flames due to more light H radical in stratified flames burned gas. The flame speed is higher for case with larger stratification gradient. Contrary to positive gradient cases, the flame speeds of stratified flames with fuel lean-to-rich as well as with temperature low-to-high are slower than homogeneous flames. The flame propagation accelerates with increasing hydrogen mole ratio due to higher H radical concentration, which indicates that chemical effect is more significant than thermal effect. Additionally, flame displacement speed does not match laminar flame speed due to the fluid continuity. Laminar flame speed is the superposition of flame displacement speed and flow velocity.     

大型轴类工件喷气冷却流动和传热特性的数值模拟

以某喷气冷却装置为参考,对缝隙喷气冷却大型轴类工件展开研究.通过数值模拟研究了具有两相对缝隙的大型轴类工件(直径D=1 000～3 000 mm)喷气冷却装置内的漉动传热特性,对比数值计算结果与实验数据验证了模型预测的准确性,探讨了大型喷气冷却装置内部气体的流动特点,分析了双缝隙喷气冷却轴类工件的传热规律,获得了轴类工...     

A study on acoustically modulated bunsen flame and its impingement heat transfer

This study experimentally examines acoustic-field-flame-interaction by using a low-power loudspeaker to actuate the oscillation of a Bunsen flame. It is observed that under acoustic forcing, the flow dynamics are altered, different patterns of the flame front are triggered, and both flame temperature field and heat transfer characteristics are changed. Moreover, impingement heat transfer is found to be increased when the flame is under acoustic modulation, indicating that acoustics can be used to promote heat transfer for flame impingement heating applications.There is a threshold forcing frequency of 300 Hz, beyond which no interaction between the sound and flame exists. The response of the flame to acoustic excitation exhibits a double-cone structure to naked eyes, and is found to be convectively bubbling, wrinkling and shrinking flame front under high-speed photography. The oscillating flame front height has exactly the same frequency as the sound, but the waveform is non-sinusoidal. Both symmetric and asymmetric distorted flame fronts are observed, with the former occurring at low frequencies while the latter at relatively higher frequencies.The effect of acoustic field on the thermal field is to lower the high-temperature region of the flame. Therefore, the cool core in the centre is narrowed, leading to higher local heat transfer. A ten percent increase in total heat transfer rate is obtained when the optimum nozzle-to-plate distance is coupled with the most effective forcing frequency of 50 Hz. Therefore, acoustic modulation is a feasible technique for promoting heat transfer.     

Heat transfer characteristics of an impinging inverse diffusion flame jet. Part II: Impinging flame structure and impingement heat transfer

This paper is the second part of the experimental study on exploring the feasibility of inverse diffusion flame (IDF) for impingement heating. The structures and heat transfer characteristics of an impinging IDF jet have been studied. Four types of impinging flame structure have been identified and reported. The distributions of the wall static pressure are measured and presented. The influences of the global equivalence ratio (), the Reynolds number of the air jet (Re_air), and the non-dimensional burner-to-plate distance (H/d_air), on the flame structure, and the local and averaged heat transfer characteristics, are reported and discussed. The highest heat transfer occurs when the tip of the flame inner reaction zone impinges on the plate. The heat transfer rate from the impinging IDF is found to be higher than that in the premixed flame jet due to the augmented turbulence level originated from the flame neck. This high heat transfer rate, together with its in-born advantage of no danger of flashback and low level of nitrogen oxides emission, demonstrates the blue, dual-structured, triple-layered IDF is a desirable alternative for impingement heating.     

Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion

In this study, laminar flame speeds at atmospheric pressure are accurately measured for H₂/Cl₂/N₂ mixtures at different equivalence ratios and N₂ mole fractions by the counterflow flame technique. A kinetic mechanism based on recently published and evaluated rate constants is developed to model these measured laminar flame speeds as well as the literature data on the concentrations of H₂, Cl₂, and HCl species in flat-burner flames and the ignition delay times from shock tube experiments. The kinetic model yields satisfactory comparison with these experimental data, and suggests that the reactions involving excited HCl(v) species and energy branching are not of substantial significance in combustion situations, and that the use of accurate elementary rate constants is instead crucial to the accuracy of the reaction mechanism.     

Heat transfer predictions with a cubic k-ε model for axisymmetric turbulent jets impinging onto a flat plate

Local heat transfer in turbulent axisymmetric jets, impinging onto a flat plate, is predicted with a cubic k-ε model. Both the constitutive law for the Reynolds stresses and the transport equation for the dissipation rate ε contribute to improved heat transfer predictions. The stagnation point value and the shape of the profiles of the Nusselt number are well predicted for different distances between the nozzle and the flat plate. Accurate flow field predictions, obtained with the presented turbulence model, are the basis for the quality of the heat transfer results. The influence of the nozzle-plate distance on the stagnation point Nusselt number, is also correctly captured. For a fixed nozzle-plate distance, the influence of the Reynolds number on the stagnation point heat transfer is correctly reproduced. Comparisons are made to experimental data and to results from a low-Reynolds standard k-ε model [1] and the v²-f model [2].