Naphthalene Sublimation Technique Research Articles

Convective heat transfer correlation for a single surrogate firebrand and a simplified firebrand pile on a flat plate using naphthalene sublimation in heated air flow

Previous studies have shown that most structure ignitions in wildland urban-interface fires are due to firebrand deposition and ignition. The heat transfer mechanisms involved in firebrand deposition need further study and characterization for better understanding of the firebrand ignition process. In particular, convective heat transfer correlations over a single firebrand and a pile of firebrands are lacking. Using the heat-mass transfer analogy, naphthalene sublimation experiments were conducted to determine convective heat transfer correlations for a single naphthalene cylinder (a surrogate firebrand) and an idealized three-firebrand pile resting on flat plates from mass loss measurements. These experiments were conducted in a wind tunnel using a heated (50 °C) or room temperature air flow (0.5 m/s to 2.1 m/s). There was good agreement between the Nusselt number correlation obtained using heated air and results with unheated airflows. Experiments using heated airflow reduced the experimental run times and uncertainty in mass loss measurements significantly. In general, the single firebrand had higher Nusselt numbers than the individual firebrands in the pile. In the three-firebrand pile, the firebrand at the top of the pile exhibited the highest heat transfer. The naphthalene sublimation technique can be easily extended to obtain convective heat transfer correlations for various firebrand geometries and configurations.

Stagnation Point Heat Transfer to an Axisymmetric Impinging Jet at Transition to Turbulence

Abstract A round air jet issuing from a long straight pipe and impinging perpendicularly onto a heated flat plate was investigated experimentally. The Reynolds number (Re) covered the fully laminar, transitional, and fully turbulent regimes—Re = 850–15,400. The main focus of this investigation was the transition regime, which occurred at Re = 2250–3010. Various measurements were recorded during the experiments using a hot-wire anemometer, an infrared thermometer, and a thermopile heat flux sensor; the mass transfer was measured using the naphthalene sublimation technique. The stagnation point heat transfer was correlated to the laminar and turbulent regimes in the form of the stagnation point Nusselt number, Nu0 = CRemPr0.4, where the exponent m = 0.50 and 0.55, respectively. The Nu0–Re relationship exhibited nonmonotonic behavior (decrease) in the transition regime. Two counteracting mechanisms occur during transition—jet core shortening and an increase in velocity fluctuation; the former reduces Nu0, whereas the latter increases it.

Effects of tip-bleed holes on two-pass channel on heat transfer with various aspect ratios

The heat transfer characteristics in 180-degree turn region of two-pass channel with three different aspect ratios were investigated as coolant being extracted through tip-holes. The naphthalene sublimation technique was used for the heat/mass transfer measurements from the internal ceiling surface of the 180-degree turn region. The channel aspect-ratios of 1:1, 2:1, and 5:1 while keeping the same hydraulic diameter were studied. The inlet Reynolds number was set at 10000 that matches an actual land-based gas turbine. The tip-extraction mass flow rate was set at 10% of the total inlet mass flow rate. The experimental results showed that the heat/mass transfer is improved with the coolant extraction from tip-holes for all test cases. The improvement of thermal performance on tip surface is ranged from 10.8% to 67.0% compared to the no coolant extraction cases. The improvement was highest for the case of 1:5 aspect ratio as the extraction helped to minimize the region of local recirculation. The study proved that the tip-hole extraction could be effectively designed to improve the cooling of 180-degree turn region.

Repetitive tip convective transport and its flow physics in a large-scale turbine cascade

In order to understand repetitive convective transport and its flow physics over the flat tip of a turbine blade, high-spatial resolution measurements of thermal load and aerodynamic loss are performed in a large-scale cascade. The thermal load distributions measured with the naphthalene sublimation technique show that for two tip gaps of h/s = 1.0% and 2.0%, there is a repetitive change of thermal load upstream of the mid-chord, and wider tip gap makes thermal load change more severe but less repetitive. The mean distance between the neighboring peaks of thermal load in the direction normal to local flow for h/s = 2.0% is twice the one for h/s = 1.0%. Present total pressure measurements show that in the area of repetitive thermal load change for h/s = 2.0%, there exists a distinct repetitive loss change only near the tip surface. A qualitative flow model in the area of repetitive change is proposed for h/s = 2.0%. According to the flow model, the separation bubble undergoes a wave-like repetitive change in its thickness and length. Just behind the ridge of the wave-like separation bubble, low thermal load coexists with high loss, whereas just behind its valley, high thermal load coexists with low loss.

Assessment of the naphthalene sublimation technique for determination of convective heat transfer in fundamental and industrial applications

The naphthalene sublimation technique is an experimental method for indirectly determining convective heat transfer. The technique is here assessed for two different configurations: the local heat transfer distribution for a circular air jet impinging normal to a flat surface, and the heat transfer occurring in the stator core of an electric generator model. The turbulent impinging jet is fully developed. Two Reynolds numbers based on the nozzle exit condition, 15000 and 23000, and two nozzle diameter distances from the jet exit to the surface, 6 and 8, are considered. For the generator turbulent internal flow with Reynolds number of 4100 is considered, based on the hydraulic diameter of stator ventilation ducts. Modern surface scanning methods and imprints of the naphthalene specimens were used for measuring the naphthalene sublimation rate. The impinging jet results are compared with experimental data found in the literature. Results from the generator model and numerical simulations are compared. For the impinging jet, the results show agreement with the already published experimental data sets. For the generator model, heat transfer results from experiments differ by around 13% compared to numerical results if a scanning of the surface is used for measuring the naphthalene sublimation and around 5% if weights are used for measuring the sublimation rate. Therefore, the results depend on the way the sublimation rate is quantified. From this study, it is possible to affirm that with advanced scanning procedures, the heat transfer can be resolved with very small naphthalene sublimation in cases of both fundamental and complex industrial applications such as electric generators.

Lateral edge effects on heat/mass transfer on a finite width surface within a turbulent boundary layer

Numerical simulations of the local heat/mass transfer on a finite width surface within a turbulent boundary layer are presented. Different approaches to the RANS modelling of the turbulent heat/mass flux are compared to Large Eddy Simulations (LES). Mass transfer experiments conducted with the naphthalene sublimation technique are used as validation. The isotropic eddy viscosity model, Simple Gradient Diffusion Hypothesis (SGDH), is shown to underestimate the span-wise effects. Its anisotropic extension, Generalized Gradient Diffusion Hypothesis (GGDH), improves the prediction, but still does not account for near-wall contribution in strongly dissimilar velocity and temperature/concentration fields, even in combination with a wall-sensitive second-moment closure model such as the Elliptic Blending Reynolds Stress Model (EB-RSM). A more complete turbulent heat flux model based on the elliptic blending approach, the Elliptic Blending GGDH (EB-GGDH) presents very good agreement with LES and with the experiments, confirming the need for more advanced turbulent heat flux modelling in applications with strong three-dimensional effects.

Characterization of Impingement Heat/Mass Transfer to the Synthetic Jet Generated by a Biomimetic Actuator

Two biomimetic synthetic jet (SJ) actuators were designed, manufactured, and tested under conditions of a jet impingement onto a wall. Nozzles of the actuators were formed by a flexible diaphragm rim, the working fluid was air, and the operating frequencies were chosen near the resonance at 65 Hz and 69 Hz. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, jet momentum flux measurement using a precision scale, hot-wire anemometry, and mass transfer measurement using the naphthalene sublimation technique. The results demonstrated possibilities of the proposed actuators to cause a desired heat/mass transfer distribution on the exposed wall. It was concluded that the heat/mass transfer rate was commensurable with a conventional continuous impinging jets (IJs) at the same Reynolds numbers.

Detailed heat transfer and friction factor characteristics in a rectangular duct with alternate solid and converging-slit ribs

ABSTRACTLiquid crystal thermography and pressure drop measurements have been carried out to study the heat transfer and frictional characteristics in a rectangular duct with solid ribs (C1), converging slit-ribs (C2), and alternate solid-slit ribs (C3) mounted transversely on the bottom wall, where C2 carries a continuous converging-slit in the flow direction. Effect of rib configurations, and rib pitch to height ratios (6, 8, 10, and 12) has been investigated at Re of 9400, 26160, 42500, and 58850. Results show that converging-slit considerably enhances the heat transfer rate in the downstream vicinity, and help in obviating the local hot spot formation.Abbreviations: LCT: Liquid crystal thermography; HTC: Heat transfer coefficient; LHI: Laser holographic interferometry; NST: Naphthalene sublimation technique; IR: Infrared; TPF: Thermo-hydraulic performance; PIV: Particle image velocimetry.

The convective heat transfer characteristics on outside surface of vehicle brake disc

The brake disc of a high-speed train can produce huge heat in the process of steep brake or parking brake. Thus the heat transfer characteristics of the high-speed train brake discs are the basic information needed to design them. The main works of the present paper are to establish a flow model that is similar to the actual brake disc working conditions, and to obtain the local and average convective heat transfer characteristics of the outside surface of vehicle break disc experimentally. The naphthalene sublimation technique of heat/mass transfer analogy method is used. The significant result is that the diameter ratio of the brake disc to the wheel determines the convective heat transfer characteristics on the vehicle's brake disc outside surface. Due to the influence of the ground, the average Nusselt number on the surface of a break disc is larger than that of a rotating disc in crossflow at the same Reynolds number. The local Nusselt number on the surface of a break disc decreases along the radius direction. The correlations of the local and average Nusselt numbers, Reynolds number, and the diameter ratio of brake disc to wheel are established. The average Nusselt number is larger than that on the surface of a rolling wheel at the same Reynolds number.

Experimental Study of Convective Heat Transfer in Grille-sphere Composite Structured Packed Bed

Randomly packed beds are widely used in practice due to their low cost and ease of use, however, the pressure drops in such packed beds are usually high. Compared with randomly packed beds, structured packed beds have a relatively lower pressure drop, while the heat transfer is weakened, and their packing form is hard to achieve. In the present paper, a grille-sphere composite structured packed bed (GSCSPB) is designed and the pressure drop together with the heat transfer in GSCSPB is experimentally investigated by using the naphthalene sublimation technique. Firstly, it is found that, the pressure drop in GSCSPB is a little higher than that of simple cubic (SC) packing and is much lower than that of randomly packed bed. Secondly, it shows that the particle-to-fluid heat transfer coefficient (hsf) in GSCSPB would be improved by 94% of SC packing, and can be 76% of that in randomly packed bed. That shows a highest overall heat transfer performance of GSCSPB. Finally, it reveals that the grille-to-fluid heat transfer coefficient (hgf) in GSCSPB is 1.05-1.23 times than hsf in GSCSPB, which indicates a huge potential for the heat transfer capacity in GSCSPB.

Modeling and Measurements of Heat/Mass Transfer in a Linear Turbine Cascade

Measurements of the mass/heat transfer coefficients on the blade and end wall surfaces of a linear turbine cascade are compared to numerical predictions using the standard shear stress transport (SST) closure and the SST model in combination with the Reθ–γ transition model (SST-TRANS). Experiments were carried out in a wind tunnel test section composed of five large-scale turbine blades, using the naphthalene sublimation technique. Two cases were tested, with exit Reynolds number of 600,000 and inlet turbulence values of 0.2% and 4%, respectively. The main secondary flow features, consisting of the horseshoe vortex system, the passage vortex, and the corner vortices, are identified and their influence on heat/mass transfer is analyzed. Numerical simulations were carried out to match the conditions of the experiments. Results show that large improvements are obtained with the introduction of the Reθ–γ transition model. In particular, excellent agreement with the experiments is found, for the whole spanwise extension of the blade, on the pressure surface. On the suction surface, performance is very good in the highly three-dimensional region close to the end wall, but some weaknesses appear in predicting the location of transition in the two-dimensional region. On the end wall surface, the SST model in combination with the transition model produces satisfactory results, greatly improved compared to the standard SST model.

Heat/mass transfer over the cavity squealer tip equipped with a full coverage winglet in a turbine cascade: Part 1 – Data on the winglet top surface

Heat/mass transfer characteristics on the winglet top surface for the cavity squealer tip equipped with a full coverage winglet has been investigated experimentally with the variation of h/s (tip gap height-to-span ratio) by employing the naphthalene sublimation technique. For a squealer rim height-to-span ratio of hst/s=3.75% and a winglet width-to-pitch ratio of w/p=10.55%, the tip gap is changed to be h/s=0.34%, 0.68%, 1.02%, 1.36%, and 1.70%. On the winglet top surface, high local heat/mass transfer rate is found (i) at the sites of the leading edge tip gap vortices, (ii) in the flow separation/reattachment area on the pressure-side winglet, (iii) in the upstream outflow area on the suction-side winglet, (iv) in the downstream outflow area on the suction-side winglet, and (v) in the area where the cavity fluid is discharged toward the trailing edge. On the other hand, very low local heat/mass transfer rate is observed in two separate areas on the suction-side winglet upstream of the mid-chord. In the case of very low h/s, the two separate low heat/mass transfer areas cannot be identified. Instead, a new high heat/mass transfer area is found near the mid-chord on the suction-side winglet. Average heat/mass transfer rate on the winglet top surface tends to increase consistently with increasing h/s, but it is always lower than that on the tip surface of the plane tip with no winglet.

Heat/mass transfer over the cavity squealer tip equipped with a full coverage winglet in a turbine cascade: Part 2 – Data on the cavity floor

Heat/mass transfer characteristics on the floor of the cavity squealer tip equipped with a full coverage (FC) winglet has been investigated experimentally with the variation of h/s (tip gap height-to-span ratio) by employing the naphthalene sublimation technique. For a squealer rim height-to-span ratio of hst/s=3.75% and a winglet width-to-pitch ratio of w/p=10.55%, the tip gap is changed to be h/s=0.34%, 0.68%, 1.02%, 1.36%, and 1.70%. In this study, the cavity floor is divided into four specific regions based on local heat/mass transfer rate, and the effects of h/s on local heat/mass transfer rate are fully discussed. The results show that average heat/mass transfer rate on the floor of the cavity squealer tip with the FC winglet increases steeply with the increment of h/s. This average value is found lower in the case of low h/s but higher in the case of high h/s, in comparison with that for the cavity squealer tip with no winglet. However, it is always lower than that on the plane tip surface with no winglet. For the cavity squealer tip with the FC winglet, average heat/mass transfer rate on the cavity floor is lower than that on the winglet top surface regardless of h/s, but the discrepancy between them tends to decrease rapidly as h/s increases.

Experimental investigation of fluid flow and heat transfer in a randomly packed bed of sinter particles

Fluid flow and heat transfer in a randomly packed bed of sinter particles (RPBSP) is experimentally investigated. Based on the Chilton–Colburn analogy, heat transfer performance is obtained by the naphthalene sublimation technique. The sinter particles, which are coated with naphthalene, are sublimated to obtain the mass transfer characteristics. The results of pressure drop (Δp) in the RPBSP show that most experimental values are within the ±20% deviations of the modified Ergun’s equation. The results of Nusselt number (Nu) indicate that, the heat transfer characteristics of the randomly packed bed of the irregular particles are different from that of regular surface particles. Slopes of heat transfer correlations of the RPBSP are larger than these with spheres. Based on the experimental investigations, we employ genetic algorithm (GA) to derive heat transfer correlations. It is found that the GA approach can correlate Nu quite well and two new correlations are obtained to describe convective heat transfer in the RPBSP. Comparisons between correlations developed in the present work and other correlations are also conducted. Lastly, effects of sphericity and porosity are included in a heat transfer correlation when considering physics and heat transfer characteristics. The investigation provides thoughts for dealing with the packed beds with irregular particles, as well as the newly obtained correlations for construction of the mathematical model in sinter cooling bed and design of new cooling equipment.

Effects of squealer rim height on heat/mass transfer on the floor of cavity squealer tip in a high turning turbine blade cascade

The effects of hst/s (squealer rim height-to-span ratio) on heat/mass transfer rate on the floor of the cavity squealer tip have been investigated in a high turning turbine blade cascade by employing the naphthalene sublimation technique along with oil film flow visualizations. Tested squealer rim heights are hst/s=0.00% (plane tip), 0.94%, 1.88%, 3.75%, and 5.63% for a tip gap height-to-span ratio of h/s=1.02%. For comparison purpose, the tip gap height is changed from h/s=0.34% to 1.70% for hst/s=3.75%. Heat/mass transfer rate on the cavity floor upstream of the mid-chord is affected by the leading edge tip gap vortices as well as by the reattachment of the incoming tip leakage flow to the cavity floor for lower hst/s, as in the plane tip case, whereas it is influenced mainly by the impingement of the incoming tip leakage flow onto the cavity floor near the leading edge for higher hst/s. On the other hand, heat/mass transfer rate downstream of the mid-chord is determined by the downwash flow which is entrapped by the suction-side squealer rim. With increasing hst/s, average heat/mass transfer rate on the cavity floor decreases steeply at first and then decreases mildly in the same manner as over-tip leakage loss. Average heat/mass transfer rate on the cavity floor is more sensitive to the squealer rim height than to the tip gap height.

The Heat Transfer Characteristics of Rolling Wheel and the Characteristic Length Determining Them

The convective heat transfer characteristics on the surface of a rolling wheel are investigated using the naphthalene sublimation technique. Five sizes of rolling wheel are selected in the experiments. The local and average Nusselt numbers are obtained. The results reveal that if the wheel radius is used as the characteristic length, the relationship between Nusselt number and Reynolds number is dependent on the wheel radius. This indicates that the wheel radius is not the characteristic length to determine the dimensionless convective heat transfer characteristics of the rolling wheel. Thus, a newly defined characteristic length is provided. For different radii of the wheel, the relationships between Nusselt number and Reynolds number based on this length collapse into one reasonable correlation. The correlation not only enriches the insight of convective heat transfer on rolling wheel but also extends the applicability of the present experimental data.

Numerical simulation of slit wall effect on the Taylor vortex flow with radial temperature gradient

Abstract Numerical simulation was applied to investigate the Taylor vortex flow inside the concentric cylinders with a constant radial temperature gradient. The reliability of numerical simulation method was verified by the experimental results of PIV. The radial velocity and temperature distribution in plain and 12-slit model at different axial locations were compared, and the heat flux distributions along the inner cylinder wall at different work conditions were obtained. In the plain model, the average surface heat flux of inner cylinder increased with the inner cylinder rotation speed. In slit model, the slit wall significantly changed the distribution of flow field and temperature in the annulus gap, and the radial flow was strengthen obviously , whichpromote d the heat transfer process at the same working condition. Keywords : Taylor vortex flow, Slit wall, temperature gradient, heat transfer, numerical simulation 1. Introduction In 1923, Taylor[1] discovered the secondary flow existed in the annulus gap between two relatively rotating concentric cylinders, which is called Taylor-Couette flow. Since then, a lot of scholars carried out in-depth research on it. Yang[2] studied the transition process of Taylor-Couette flow by flow visualization method, the aluminum particles were added to investigate the stability of flow field, they found as the rotation speed increased, the aluminum particles have the axial motion before the transition to Taylor vortex flow. Kedia et al.[3] studied the effects of heat transfer on Taylor-Couette flow, and found the heat transfer decreased with Grashof number in axisymmetric Taylor vortex flow regime, and increased with Grashof number after the flow becomes non-axisymmetric. Lee and Minkowycz [4] studied heat transfer characteristics by using the naphthalene sublimation technique in the annular gap between two short concentric cylinders which had either two walls or one plain plain and one axially slit wall, and yielded qualitative information regarding heat transfer but did not address the flow phenomena inside the annular gap. Liu[5] studied the axial wall slits effect on the Taylor vortex flow in the gap between two concentric cylinders by Particle Image Velocimetry, and found the existence of slit wall accelerated the transition process.

SIMULATION OF HEAT TRANSFER FROM CANOPY SURFACES USING LOW-REYNOLDS NUMBER K-ε MODEL

This study focuses on the Convective Heat Transfer Coefficient (CHTC) from urban building surfaces by numerical simulation. The heat transfer effects because of various geometrical and physical properties of urban areas exhibits a differential heating and uncomfortable environment compared to rural regions called as Urban Heat Island (UHI) phenomena. Investigation of Convective heat transfer coefficient becomes more important in the study of urban heat island phenomena. Experimental simulation of urban area with various urban canopy cases in thermally stratified wind tunnel is employed for the heat transfer kind of investigations in urban area. But, it is not an easy task in wind tunnel experiments to evaluate local CHTC, which vary on individual canyon surfaces transfer such as building roof, walls and ground. Numerical simulation validated by wind tunnel experiments can be an alternative for the prediction of CHTC from building surfaces in an urban area. In our study, Water evaporation technique used in wind tunnel experiment for the evaluation of convective heat transfer coefficient and naphthalene sublimation technique conducted by other researchers are used to validate the low-Reynolds-number k-e model which was used for the evaluation of CHTC from surfaces. The calculated CFD results showed good agreement with both water evaporation technique and naphthalene sublimation experimental results. It is found that the low-Reynolds-number k-e model is reliable for the investigations pertaining to heat transfer from urban canopy.

SIMULATION OF HEAT TRANSFER FROM CANOPY SURFACES USING LOW-REYNOLDS NUMBER K-ε MODEL

This study focuses on the Convective Heat Transfer Coefficient (CHTC) from urban building surfaces by numerical simulation. The heat transfer effects because of various geometrical and physical properties of urban areas exhibits a differential heating and uncomfortable environment compared to rural regions called as Urban Heat Island (UHI) phenomena. Investigation of Convective heat transfer coefficient becomes more important in the study of urban heat island phenomena. Experimental simulation of urban area with various urban canopy cases in thermally stratified wind tunnel is employed for the heat transfer kind of investigations in urban area. But, it is not an easy task in wind tunnel experiments to evaluate local CHTC, which vary on individual canyon surfaces transfer such as building roof, walls and ground. Numerical simulation validated by wind tunnel experiments can be an alternative for the prediction of CHTC from building surfaces in an urban area. In our study, Water evaporation technique used in wind tunnel experiment for the evaluation of convective heat transfer coefficient and naphthalene sublimation technique conducted by other researchers are used to validate the low-Reynolds-number k-ε model which was used for the evaluation of CHTC from surfaces. The calculated CFD results showed good agreement with both water evaporation technique and naphthalene sublimation experimental results. It is found that the low-Reynolds-number k-ε model is reliable for the investigations pertaining to heat transfer from urban canopy.

SIMULATION OF HEAT TRANSFER FROM CANOPY SURFACES USING LOW-REYNOLDS NUMBER K-ε MODEL

This study focuses on the Convective Heat Transfer Coefficient (CHTC) from urban building surfaces by numerical simulation. The heat transfer effects because of various geometrical and physical properties of urban areas exhibits a differential heating and uncomfortable environment compared to rural regions called as Urban Heat Island (UHI) phenomena. Investigation of Convective heat transfer coefficient becomes more important in the study of urban heat island phenomena. Experimental simulation of urban area with various urban canopy cases in thermally stratified wind tunnel is employed for the heat transfer kind of investigations in urban area. But, it is not an easy task in wind tunnel experiments to evaluate local CHTC, which vary on individual canyon surfaces transfer such as building roof, walls and ground. Numerical simulation validated by wind tunnel experiments can be an alternative for the prediction of CHTC from building surfaces in an urban area. In our study, Water evaporation technique used in wind tunnel experiment for the evaluation of convective heat transfer coefficient and naphthalene sublimation technique conducted by other researchers are used to validate the low-Reynolds-number k-ε model which was used for the evaluation of CHTC from surfaces. The calculated CFD results showed good agreement with both water evaporation technique and naphthalene sublimation experimental results. It is found that the low-Reynolds-number k-ε model is reliable for the investigations pertaining to heat transfer from urban canopy.