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.     

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.     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL LAMINAR,SQUARE TWIN-JET IMPINGEMENT ON A FLAT PLATE,FLOW STRUCTURE,AND HEAT TRANSFER

The flow and heat transfer characteristics of impinging laminar square twin jets have been investigated numerically through the solution of three-dimensional Navier-Stokes and energy equations in a steady state. The simulations have been carried out for jet-to-jet spacings of 4, 6, and 8 and for nozzle-exit-to-plate distances between 0.25D and 5D. The calculated results show that the flow structure of square twin jets impinging on a heated plate is strongly affected by the jet-to-plate distance. In addition, for very small jet-to-plate distances (L z , 0.25D), no upwash fountain flow can form at the collision point where the jets are merely diverted in the transverse direction. For such nozzle-to-plate distances the wall jet fills the whole gap between the plates with no vortex motion around the twin jets.     

Experimental investigation on the heat transfer of an impinging inverse diffusion flame

This paper presents the results of an experimental study on the heat transfer characteristics of an inverse diffusion flame (IDF) impinging vertically upwards on a horizontal copper plate. The IDF burner used in the experiment has a central air jet surrounded circumferentially by 12 outer fuel jets. The heat flux at the stagnation point and the radial distribution of heat flux were measured with a heat flux sensor. The effects of Reynolds number, overall equivalence ratio, and nozzle-to-plate distance on the heat flux were investigated. The area-averaged heat flux and the heat transfer efficiency were calculated from the radial heat flux within a radial distance of 50 mm from the stagnation point of the flame, for air jet Reynolds number (Re_air) of 2000, 2500 and 3000, for overall equivalence ratios (Φ) of 0.8–1.8, at normalized nozzle-to-plate distances (H/d_IDF) between 4 and 10. Similar experiments were carried out on a circular premixed impinging flame for comparison.It was found that, for the impinging IDF, for Φ of 1.2 or higher, the area-averaged heat flux increased as the Re_air or Φ was increased while the heat transfer efficiency decreased when these two parameters increased. Thus for the IDF, the maximum heat transfer efficiency occurred at Re_air = 2000 and Φ = 1.2. At lower Φ, the heat transfer efficiency could increase when Φ was decreased. For the range of H/d_IDF investigated, there was certain variation in the heat transfer efficiency with H/d_IDF. The heat transfer efficiency of the premixed flame has a peak value at Φ = 1.0 at H/d_P = 2 and decreases at higher Φ and higher H/d_P. The IDF could have comparable or even higher heat transfer efficiency than a premixed flame.     

Numerical study of the radiation effect on the jet array impinging heat transfer in a feeding pipe

The present study deals with the turbine casing radiation effect on the impinging cooling encountered in the blade tip active clearance control (ACC) system of aero-engine turbine. To this end, numerical simulations are carried out for a simplified model, that is, a pipe with a single row of impinging jets. The effects of the nozzle-to-plate distance to the diameter of the impinging hole (H/d?=?2–8), the number of the holes (n?=?17–68), the impinging wall temperature (T_p?=?400–800?K), and the inlet Reynolds number (Re?=?5,000–20,000) on the flow and heat transfer are investigated. Analysis is performed on the radiation heat transfer effects on the corresponding flow fields and surface heat flux distributions. The results indicate that, with the radiation included in the simulations, the mass flow rate of the cooling jet near the end of the pipe decreases significantly under the conditions of high wall temperature and small nozzle-to-plate distance. Radiation heat transfer should be accounted for in the numerical study for the casing cooling as it affects the flow and heat transfer remarkably. When the nozzle-to-plate distance is relatively large, such as H/d is larger than 8, the radiative heating leads to uniform heat flux and the radiative heating can suppress the uneven distributions of the heat flux.     

Thermal performance of a premixed impinging circular flame jet array with induced-swirl

An array of three identical premixed butane–air-fired impinging circular flames with induced-swirl operating at low-pressure and low-Reynolds-number was developed. A swirling motion was imparted successfully to the flame by forcing the butane/air mixture through a specially designed burner assembly before ignition. The burner assembly consisting of a conical base and a nozzle tube into which a cylindrical bar fabricated with three spiral channels was inserted. Its thermal performance was compared with that of a similar impinging flame jet system without induced-swirl. Effects of varying the Reynolds number and the equivalence ratio of the butane/air mixture and the nozzle-to-plate distance on the thermal performance of each of these two impinging flame jet systems were studied. Experiments were conducted with different combinations of Reynolds number, equivalence ratio and nozzle-to-plate distance. In the present investigation, the Reynolds number ranged from 500 to 2500, the equivalence ratio ranged from 1.0 to 1.8 and the nozzle-to-plate distance ranged from 20 mm to 30 mm. To facilitate comparison, flame shapes of both impinging flame jet systems were also visualized by a high speed digital camera system. The comparison showed that the array of three small-scale, low-pressure and low-Reynolds-number premixed butane–air-fired impinging circular flame jets could enhance its thermal performance, with respect to heat transfer characteristics and blow-out limits, by incorporating an induced-swirl. The performance enhancement increased with increasing Reynolds number or equivalence ratio, but decreased with increasing nozzle-to-plate distance.     

Influence of a pulsation on heat transfer and flow structure in submerged impinging jets

An experimental investigation on pulsating impinging jets has been performed. The effect of the pulsation on the flow structure and heat transfer have been investigated. Frequency and amplitude were varied separately and the effect of each parameter was examined for different Reynolds numbers and nozzle-to-plate distances.The jet was found to become broader and the core jet length smaller with the pulsation. The reason for this behavior is that pulsation enhanced entrainment of air into the jet, which results in a change of mean velocity of the jet. Nevertheless, the behavior at lower frequencies (up to 140 Hz) is still quasisteady. This means that the amplitude of the pulsation behaves similar to the mean velocity of the jet, that the shapes of the velocity profiles are comparable to steady jets and that the jet behavior is independent of frequency.At moderate frequencies heat transfer is only affected by the pulsation when nozzle-to-plate distance and amplitude are large enough. At small nozzle-to-plate distances enhanced entrainment has no influence and no difference between steady and pulsating jets can be recognized. At large nozzle-to-plate distances entrainment increases and jet velocity reduces. This yields a reduction of heat transfer in the stagnation point of up to 50%.But besides of this effect of enhanced entrainment a theoretical limit could be determined, above which the jet is not anymore quasisteady. Above Sr = 0.2 heat transfer is affected by the pulsation also at small nozzle-to-plate distances. At this frequency boundary layer is also affected by the pulsation. This yields increased heat transfer coefficients at the stagnation point. For larger nozzle-to-plate spacings this effect is superposed by the reduction of heat transfer due to increased entrainment, resulting in a strong decrease of heat transfer coefficient.     

Local heat transfer and recovery factor with impinging free-surface circular jets of transformer oil

Measurements were made to investigate the local behavior of the recovery factor and the heat transfer coefficient with free-surface circular jets. The experiments were performed with transformer oil jets impinging on a vertical constant-heat-flux surface from small pipe and orifice nozzles of 1 mm diameter in the ranges of Re = 183–2600 and Pr = 82–337. Large values of recovery factor over 20 were recorded with medium jet velocity about 20 m s⁻¹. Radial distribution of the recovery factor was determined and expressed in empirical equations. The heat transfer coefficient at stagnation point was found to be nearly independent of nozzle-to-plate spacing, but proportional to the square root of the jet Reynolds number. Profiles of local heat transfer coefficients were obtained and correlated. Based on the local measurements, integral average heat transfer coefficients were obtained and correlated.     

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.     

Parametric Study and Optimization of Staggered Inclined Impinging Jets on a Concave Surface for Heat Transfer Augmentation

The characteristics of the fluid flow and heat transfer of staggered inclined impinging jets on a concave surface have been investigated numerically using three-dimensional Reynolds-averaged Navier-Stokes analysis using the shear stress transport turbulence model. Shape optimization of the impinging jet has been performed with a weighted-average surrogate model. A constant temperature condition has been applied to the concave surface. The inclination angle of the staggered jet nozzles and the distance between the jet nozzles are chosen as the design variables, and their effects on the heat transfer performance have been evaluated. It is found that the overall heat transfer increases with the pitch of vertical jet nozzles, and the staggered inclination of jet nozzles improves the heat transfer on the concave surface. For the optimization of the impinging jet, the area-averaged Nusselt number on the concave surface is set as the objective function. Latin hypercube sampling is used to determine the training points as a design of experiment, and the surrogate model is constructed using the objective function values at the training points. Sequential quadratic programming is used to search for the optimal point from the constructed surrogate model. Through the optimization, the heat transfer performance has been improved by nearly 60% compared to the reference design.     

Boiling Heat Transfer of Multiple Impinging Jets on a Hot Moving Plate

In this research, boiling heat transfer on a hot moving plate caused by multiple impinging water jets in multiple jet rows is studied. An inverse heat conduction code is developed to analyze the readings of thermocouples that are implemented inside the plate in order to find the surface values of temperature and heat flux. Effects of nozzle stagger, plate velocity, and jet line spacing are studied. Nozzle stagger is found to affect the uniformity of heat transfer across the width of the plate. Jet line spacing can affect the heat transfer between two adjacent jet rows. Plate speed is important only in the higher entry temperatures and in the impingement zone.     

Thermal characteristics of a premixed impinging circular laminar-flame jet with induced swirl

A swirling flow has been induced in a premixed gas-fired impinging circular flame jet by adding two tangential air flows to the main axial air/fuel flow. The flame jet system was considered to be small-scale and operated under low-pressure, laminar flow conditions. The effects of Reynolds number of the air/butane mixture and nozzle-to-plate distance on the heating performance of the flame were studied and compared with the heat-flux distributions on an impingement plate under different operating conditions. The whole investigation was conducted under the stoichiometric air/fuel condition (i.e., equivalence ratio, Φ = 1) with the Reynolds number being varied from 800 to 1700, and nozzle-to-plate distance being selected between 1.5 and 4.0. The introduction of swirl to small-scale, low-pressure, laminar premixed gas-fired impinging circular flame jets is the method for enhancing their thermal performances. The heat-flux distribution on the impingement plate was more uniform and the flame temperatures essentially higher when compared with a similar flame jet system without induced swirl.     

Convective heat transfer from a partially premixed impinging flame jet. Part I: Time-averaged results

Axial and radial profiles of time-averaged local heat fluxes of methane-air jet flames impinging normal to a cooled plate are reported, as functions of equivalence ratio, Reynolds number, and nozzle-plate spacing. Time-resolved behavior for these conditions is examined in the companion paper, Part II. Flame structure was studied visually and photographed. Both premixed and diffusion flame behavior was observed. Nozzle-stabilized flames revealed a stable, axisymmetric flame structure at nozzle-plate spacings less than 14 diameters. At greater nozzle-plate spacings, buoyancy-induced instabilities caused the flame to oscillate visibly. Lifted flames exhibited varied flame structures dependent upon the Reynolds number, equivalence ratio, and nozzle-plate spacing, stabilizing in the free jet, at the stagnation zone, or downstream in the wall jet. Local heat flux measurements made in the stagnation zone and along the plate adjacent to the wall jet flame revealed correlation of the local heat flux to the flame structure. Negative heat fluxes resulted from cool gases impinging on the hotter plate. The magnitude of positive heat fluxes depended on the proximity of the flame to the sensor surface, the rate of heat release, and the local molecular and turbulent transport.     

Heat transfer to impinging round jets with triangular tabs

Experiments were performed to characterize the heat transfer enhancement produced by adding arrays of triangular tabs to the exit of turbulent round impinging jets issuing from a long pipe. For small nozzle-to-plate distances the local heat transfer was increased more than 25% in a series of distinct regions surrounding the impingement region. The largest increase in the average Nusselt number occurred for a nozzle-to-plate distance of approximately 4 diameter. In this case, the average Nusselt number was increased by 20% for the impingement region but only approximately 10% for the region with a radius of 3 jet diameters. Measurements of the velocity field were performed in free jets with tab arrays to investigate how the tabs modify the development of the flow.     

A three-dimensional numerical simulation of impinging jet arrays on a moving plate

The structure of the flow field and its effect on the heat transfer characteristics of a jet array system impinging on a moving heated plate are investigated numerically for Reynolds numbers between 100 and 400 and for steady state conditions. An array consisting of 24 square jets (3 rows × 8 columns) impinging on a moving heated flat surface is considered as a representative pattern.The simulations have been carried out for jet-to-jet spacing in the range 2D–5D and for nozzle exit to plate distance of 0.25D, where D is the jet width. The velocity ratios of the moving heated plat to the jet velocity (R_m = u_p/u_j) used are in the range 0.25–1.0. The obtained results were compared with published data for the case of fixed heated plate (R_m = 0.0). The results show that the streamwise profile of the Nusselt number exhibit strong periodic oscillations, spatially. The amplitude of the periodic oscillations of the Nusselt number is attenuated as one proceeds in the downstream direction. For such small nozzle-to-plate spacing used, the results show that the ratio R_m has no effect on the oscillations of Nusselt number.     

Analysis of the heat transfer of an impinging laminar flame jet

Flame jet impingement is used in many industrial processes. In this paper an analytical expression is derived for the heat flux of a laminar flame impinging on a flat plate, where the flame jet is approximated by a hot inert jet with the position of the tip of the flame taken equal to the nozzle position. The heal flux in this expression is dependent on the nozzle-to-plate spacing, in contradiction to existing (semi-analytical) relations. The geometry is divided in a region far from the plate and a region dose to the plate. For both regions the velocity profiles are calculated using only the dominant terms of the balance equations. Subsequently these profiles are linked to each other at the boundary between the two zones. Implementing the resulting velocity profile for the complete geometry in the energy equation and integrating over the whole domain results in an expression for the heat flux from the flame to the plate at the hot spot. Numerical calculations show very good agreement with the results of the analytical derivation.     

Premixed flame impingement heat transfer with induced swirl

Experiments were performed to study the heat transfer characteristics of a swirling premixed flame impinging vertically normal to a horizontal plate. The effects of Reynolds number (Re), equivalence ratio (Ф) and nozzle-to-plate distance (H) on the heat flux were examined. Comparisons were also made between the heat transfer behaviors of the swirling premixed flame (SPF) with a non-swirling premixed flame (PF) operating under the same conditions.Compared with the PF, the swirling flows in the SPF increase the entrainment of ambient air and induce a faster radial spreading rate of the flame jet. Therefore, the SPF provides a larger heating area and produces a more uniform radial heat flux distribution. For both the SPF and PF, the heat flux increases with Re due to the more complete combustion occurring at higher Re. For the SPF, the heat transfer increases with Ф, while it decreases with Ф for the PF because the stronger entrainment of ambient air in the SPF supports a more complete combustion. A smaller H is required for the maximum heat transfer to occur for the SPF.     

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).     

Convective heat transfer from a partially premixed impinging flame jet. Part II: Time-resolved results

This is the second of a two-part paper on heat transfer from an impinging flame jet reporting time-resolved results. Axial and radial profiles of time-resolved local heat fluxes of methane-air jet flames impinging normal to a cooled plate are reported, including the root mean square (RMS), probability distribution function (PDF), and the power spectral density (PSD) of the heat flux fluctuations as a function of equivalence ratio, Reynolds number, and nozzle-plate spacing. The RMS, PDF, and PSD of the heat flux signal from the stagnation point and along the plate revealed correlation of the local heat flux to the flame structure. Impingement heat flux from premixed nozzle-stabilized flames was characterized by small RMS fluctuations and frequency behavior indicating the formation of weak, buoyancy-driven vortex structures at the shear layer between the hot gases surrounding the flame and the ambient air. Conversely, diffusion flames were characterized by much larger RMS fluctuations and PSD’s indicating the development of much larger vortex structures. Time-resolved heat flux for lifted flames varied according to flame structure and combustion intensity. PSD magnitudes were related to the range of temperatures in the flow; greater temperature ranges produced larger heat flux variations. The contributing frequencies were related to the duration of the heat flux fluctuation; more rapid changes in heat flux produced higher frequency content.