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In this experimental research, a single‐pass solar air heater comprising a porous curved channel is investigated to reveal the scope of high thermal performance during the winter season. The investigation explores the geometrical parameters for the porous channel maintained by using steel wiremesh layers of wire diameter of 0.45 mm and pitch of 2.35 mm, and the number of layers ranging from 3 to 12, which presents the channel porosity, ϕ$$ \phi $$, from 99% to 96%, respectively. The curved porous channel offers additional fluid mixing, thermal backup, and high heat transfer area, thereby increasing the convective heat transfer to the air and consequently the thermal performance. A series of experiments were carried out under real outdoor conditions to examine the various factors such as variable channel porosity, air flow rate, and the amount of solar energy received. The findings revealed that the curved porous channel with a channel porosity of 96% results in the maximum thermal and thermohydraulic efficiencies of about 85% and 79%, respectively, and the outlet air temperature of 79°C. [ABSTRACT FROM AUTHOR]

Heat transfer characteristics of household burner flames impinging on a flat surface have been investigated experimentally. This experimental study employed the analytical inverse heat conduction procedure (IHCP) concept for obtaining the local heat flux distribution. The heat flux distribution has been studied for various operating parameters such as firing rates, separation distance, burner-cap port diameter and equivalence ratios. The average heat flux distribution for three different burner caps has been compared at similar operating conditions. The separation distance between the burner tip and target surface has significant importance in heat transfer and stability viewpoint. Results of present experimental study revealed that the maximum heat flux values occur at separation distance corresponding to 15 mm. The firing rate has been observed to significantly influence the heat flux distribution on the impinging surface. The heat flux distribution has been found to be more uniform at higher separation distances and higher firing rates. [ABSTRACT FROM AUTHOR]

Nowadays, nanorefrigerants have been focused across the globe to improve the heat transfer characteristics of cooling units. Incorporating high thermal conductivity nanoparticles to the conventional refrigerants presented broad facts such as increased heat transfer coefficients, improved pool boiling inside closed cycles, enhanced COP, reduced compressor energy consumption of domestic refrigerators, and enhanced heat transfer rate during the two-phase flow of fluids. The present article focused on comprehensive experimental and numerical research reports of nanorefrigerants and nanolubricants. The heat and mass transfer and thermophysical characters such as viscous behavior, thermal conductivity, specific heat, and density have been discussed for various facts like flow condensation of fluids, evaporation, refrigerants pool boiling, and other related processes. The results showed that the volume concentration, diameter, and length of particles have a significant and crucial impact on the heat transfer rate and characteristics of pure refrigerants. Based on the available reports, it can say that the application of nanoparticles in pure refrigerants may enhance its features by about 50 – 60%. Further, the proposed correlations available in the literature for nanorefrigerants have been well discussed. [ABSTRACT FROM AUTHOR]

Experimental investigations have been carried out for impingement heat transfer characteristics of an enclosed dual swirling flame for various enclosure configurations. Effects of the size of the enclosure (DE = 70 mm, 90 mm, 90 mm, 110 mm and 150 mm) and the level of gap between the top of the enclosure and the target surface (G = 12, 16, 20, 24 and 28 mm) have been examined at fixed other operating conditions (Re(o), H/Dh and S). In addition to this, the effects of variation in outer swirling flame Reynolds numbers (Re(o) = 7000–13,000), dimensionless separation distances (H/Dh = 2–5) and the swirl number (S = 0.86, 1.54 and 2.4) have also been examined. Impingement heat transfer characteristics strongly depend on the size of the enclosure (DE) and the level of gap (G). Larger sized enclosures (DE of 110 and 150 mm) with smaller gap levels (G of 12 and 16 mm) perform much better than small sized enclosures (DE of 70 and 80 mm) with larger gap levels (G of 24 and 28 mm). Heat transfer performance continuously increases with an increase in Re(o) in presence of an enclosure in contrast to unenclosed flames where performance deteriorates at high Re(o). The heat transfer enhancement effect in presence of enclosure is more pronounced at larger separation distances (H/Dh of 4 and 5) because of the formation of the outer recirculation zone (ORZ). The effect of the presence of enclosure on impingement heat transfer enhancement becomes more significant at higher levels of swirl. [ABSTRACT FROM AUTHOR]

Detailed flow field and heat transfer characteristics of an array of three corotating dual swirling impinging flames arranged in a triangular configuration have been investigated experimentally as well as numerically. Effect of varying separation distance and inter-jet spacing on interactions and impingement heat transfer has been studied at fixed Reynolds numbers under stoichiometric conditions. Adjacent co-swirling flames developed strong interactions resulting in high heat transfer due to increased mixing and turbulence. Bending of inner non-swirling flames takes place toward geometric center of the array due to asymmetric interactions. Numerical simulation also predicted asymmetric interactions resulting in the formation of large distorted recirculation bubble in each dual-swirling flame. Interacting co-swirling flames resulted in the formation of circulating patterns of flow at the geometric center of array. The effect of inter-jet spacing on average heat flux is observed to be more significant than separation distance. [ABSTRACT FROM AUTHOR]

Detailed flow field and heat transfer characteristics developed in a row of three co-rotating dual swirling impinging flames have been investigated experimentally as well as numerically. Impingement heat transfer and pressure distribution has been studied experimentally at different separation distances and inter-jet spacings. Inverse heat conduction procedure (IHCP) has been used for estimating heat fluxes on the front side of impingement plate. Turbulence induced mixing results in strong interactions amongst adjacent flames causing deflections of inner flames of the burners situated at sides of central burner. Numerical simulation predicted formation of asymmetric recirculation zones for side flames. Symmetric interactions taking place for central flame produced two equal recirculation lobes for central flame. Behavior of central inner flame has been observed to be dependent on the value of inter-jet spacing used. Suppression of central inner flame tend to occur at higher inter-jet spacings due to recirculating products. Impingement pressure distribution is observed to be consistent with the heat flux distribution. Averaged heat fluxes registered at the impingement plate due to the central flame are higher in magnitude to those pertaining to each of the side flames. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of counter rotating dual-swirling flames impinging on a flat surface. Effect of separation distance (H/D h = 3, 4, 6 and 8), inter-jet spacings (S/D h = 4, 7 and 10) and Reynolds number on flow interactions and impingement heat transfer have been studied under stoichiometric conditions. The inner non-swirling flames are seen to bend towards interacting side due to asymmetric interactions developed in a pair of dual-swirling flames. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting dual-swirling impinging flames. Numerical simulation predicted the deflection of inner flames to be primarily due to large distorted RCZ developed from asymmetric interactions. Comparisons of co-and-counter swirling configurations suggest that the presence of plate has a stronger influence on flow structure of impinging flames than sign of rotation. High resolution heat fluxes have been obtained using steady state 1-D heat balance from temperature readings obtained on rear side of quartz impingement plate through IR thermal camera. 1-D heat balance has been corrected for lateral conduction taking place inside the impingement plate using an iterative procedure. For counter swirl case, cross-flow leaves as a straight flow along the transverse direction due to the nature of interactions. Cross-flow causes heat flux contours in the interaction region to be asymmetric and preferential heating is observed along the transverse direction. The increase in Re (o) causes wider flame spread and severe interactions result in increased tendency for preferential heating of the impingement plate. High average heat fluxes are obtained at smallest H/D h and S/D h whereas minimum relative deviation is observed at S/D h = 10 for H/D h = 4. [ABSTRACT FROM AUTHOR]

• Irreversiblity of nanorefrigerant during flow condensation is studied. • R123-MWCNTs nanorefrigerant flows in horizontal smooth tube at different fractions. • Total entropy reduces as vapour quality, mass velocity, and MWCNTs fraction increase. • Bejan number and exergy loss decreases with increase of vapour quality. • Highest reduction in exergy destruction about 29 % is noticed for R123-MWCNTs. This experimental study examines the entropy generation due to heat transfer and frictional pressure and the exergy destruction during the condensation of an R123-MWCNTs nano-refrigerant within a horizontal tube. The effects of entropy production, Bejan number, and exergy loss are studied, together with the roles played by vapour quality, mass flux, and multi-wall carbon nanotubes (MWCNTs) concentration. The results show that the heat transfer entropy generation reduces as the vapour quality or mass flux and concentration of MWCNTs rises. In contrast, the entropy production due to frictional pressure during condensation increases with the rise in mass flux, vapour quality, and the MWCNTs fractions. The reductions in heat transfer entropy generation are high compared to the increment in frictional entropy generation, thus reducing the total entropy generation of the system. Further, nanofluid minimizes the system's Bejan number and exergy loss. The highest reduction in entropy generation and exergy loss of about 30 % and 29 %, respectively, was observed using the proposed nanofluid at a MWCNTs fraction of 0.1 % and mass velocity of 400 kg/m2s. [Display omitted] [ABSTRACT FROM AUTHOR]

An experimental study has been conducted to investigate heat transfer, friction factor and thermo-hydraulic performance characteristics of flow in a rectangular duct artificially roughened on one side with multi-gap V-down ribs combined with staggered ribs. The rectangular duct used had aspect ratio of 12 and the Reynolds number based upon the mass flow rate of air at inlet of the duct ranged from 4000 to 12,000. The rib pitch-to-height (P/e) ratio was varied from 4 to 14, rib height-to-hydraulic diameter (e/D h ) ratio from 0.026 to 0.057 and angle of attack (α) from 40° to 80° while, gap width to rib height (g/e) ratio of 1, staggered rib length to rib height ratio (w/e) of 4.5, relative staggered rib pitch (p/P) value of 0.65 and two number of gap (n) on each side of the V-leg were used as fixed parameters during the experimentation. Results show two peaks for Nusselt number corresponding to the P/e value of 6 and 12 and decrease in the Nusselt number was observed for increase in the e/D h value beyond 0.044. The maximum enhancement achieved in Nusselt number and thermohydraulic performance parameter was of 3.34 and 2.45 times respectively. [ABSTRACT FROM AUTHOR]

This paper presents thermo-hydraulic performance comparison of rib roughness under investigation, ‘V-down ribs with gap’ and similar reported rib roughness geometries used in solar air heater duct. The present rib roughness has flow-attack-angle and relative roughness height of 60° and 0.043, respectively. The duct has aspect ratio of 12 and the Reynolds number ranged from 3000 to 15,000. The roughened wall was uniformly heated while the remaining three walls were insulated. These boundary conditions correspond closely to those found in conventional solar air heaters. Five rib roughened plates having relative roughness pitch of 4, 6, 8, 10 and 12 have been tested. The Nusselt number and friction factor were found to be highest for relative roughness pitch of 8. Maximum enhancement in Nusselt number and friction factor has been found to be 2.70 and 2.86, respectively. Thermo-hydraulic performance parameter ranged from 1.27 to 1.93. Thermo-hydraulic comparison with similar rib geometries show that the present roughness geometry performs better for Reynolds number range of 3000–12,000. [ABSTRACT FROM AUTHOR]

An experimental investigation has been conducted on the heat transfer characteristics of compressed natural gas (CNG)/air dual flame (with co-swirling inner and outer flames) impinging on a flat surface. Effects of various parameters such as outer vane swirler angle ( θ o = 30°, 40° and 50°), inner vane swirler angle ( θ i = 0°, 15°, 22.5° and 30°), outer swirling flame Reynolds number ( Re o = 5000–9000), and dimensionless separation distance ( H / D h = 1–4) on heat transfer characteristics have been evaluated. The inner flame Reynolds number ( Re i ) was kept constant to 1300. A significant difference in the heat transfer characteristics has been observed in the stagnation region for different values of the θ o and θ i at different H / D h . High local and average heat fluxes have been observed at smaller separation distances. It has also been observed that the distribution of heat flux on the impingement surface was more uniform for θ o of 40° at H / D h of 2 and 3. Inducing swirl also to the inner flame has made the heat flux distribution further more uniform in the stagnation region and the effect was most favorable at low swirls ( θ i = 15°). Comparisons were also made for average heat fluxes at different radial extent of integration areas for different operating conditions. It has been found that radial extent of integration area correspond to r / D h of 5 covers most of the effective heating area of the dual impinging flame. Finally, it has been inferred from the present study that a combination of θ o of 40°, θ i of 15°, H / D h of 2 and Re o of 7000 produces the lowest percentage relative deviation and significantly high average heat transfer to the target surface for dual flame impinging on a flat surface. [ABSTRACT FROM AUTHOR]

An experimental study has been conducted to evaluate the heat transfer characteristics of CNG/air dual flame (outer swirling and inner non-swirling flame) impinging on a flat surface. Effects of variation of outer swirling flame Reynolds number (Re(o) = 3000-11,000), inner non-swirling flame Reynolds number (Re(i) = 900-1700), dimensionless separation distance (H/Dh = 1-5) and equivalence ratio of outer swirling flame (ϕ(o) = 0.8-1.2) on heat transfer characteristics have been investigated at fixed inner flame equivalence ratio (ϕ(i) = 1.0) and swirling insert helix angle of 40°. A significant effect on shape and size of the inner non-swirling flame has been observed due to central recirculation zone established by the outer swirling flame and vice versa. Impingement pressure distribution shows that the pressure peaks are the locations of peak heat fluxes on the impingement surface. Effect of presence of inner flame on heating characteristics is considerably less at larger separation distances (H/Dh ≥ 5) due to dominating effect of recirculation zone over the central axial flow. Heating characteristics showed that the heat flux distribution lines corresponding to inner flame were pushed inward towards burner axis with increase in outer swirling flame Re(o) at fixed Re(i). On the other hand, the inner boundary of the outer flame had shifted outwards with increase in Re(i) at fixed Re(o). Average heat flux imparted to the target surface increases with decrease in separation distance and increase in Re(o). There has not been much gain in the average heat transfer to the impingement surface beyond certain value of Re(o) because the rate of increase of average heat flux decreases with increase in Re(o) due to entrainment effect. Relative deviation in heat flux distribution on the impingement surface decreases with increase in separation distance. In most of the cases it again starts increasing for H/Dh > 2. In many cases, with further increase in separation distance beyond H/Dh of 3, the deviation starts continuously decreasing. Thus, it is concluded that for a dual flame impinging on a flat surface, best operating conditions prevail at moderate Re(o) (7000-9000) and H/Dh of 2. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation, thermo-hydraulic performance of rectangular ducts roughened with a new configuration of ‘V-down rib having gap’ on one wide wall is determined. Small symmetrical gap equal to rib height is created at the centre of both legs of V of continuous V-down rib. The duct has aspect ratio (AR) of 12 and the Reynolds number (Re) ranged from 3000 to 15,000. To simulate the indoor testing of solar air heater, the roughened side of rectangular duct is heated with constant heat flux electric heater while the other sides are insulated. The roughness has relative roughness height of 0.043 and relative roughness pitch of 8. Five rib roughened plates having flow-attack-angle (α) from 30° to 75° have been tested. The thermo-hydraulic performance parameter based on equal pumping power (η), friction factor (f) and Nusselt number (Nu), were found to be highest for α =60°. The results obtained at α =60° were compared with those of continuous V-down rib for same rib-roughness parameters. [Copyright &y& Elsevier]

Abstract: An experimental study has been conducted to investigate the heat transfer characteristics of compressed natural gas (CNG)/air swirling flames impinging on a flat surface. Qualitative flame structures have been studied by taking direct photographs of impinging flames. The radial temperature distribution of swirling free and impinging flames is presented at different axial heights under fixed operating conditions. Effects of dimensionless separation distance (1–6), Reynolds number (3500–6000), equivalence ratio (1–1.5) and helical vane swirler angle (0–60°) on heat transfer characteristics have been investigated. The heat transfer characteristics of swirling and non-swirling flames have been compared under similar operating conditions. A dip in the heat flux at and around stagnation point was observed in almost all cases which could be the main cause of non-uniformity even in case of heating with swirling impinging flames. The dip in heat flux becomes more pronounced at higher Reynolds number. The heat flux distribution on the impingement plate was more uniform with swirl as compared to without swirl. Also, heat flux distribution was observed to be more uniform at moderate separation distance and at larger helical vane swirl angle. Stagnation point heat flux variation remains almost constant at higher separation distances for different values of Reynolds number and equivalence ratios. There was significant decrease in average heat flux with increase in separation distance. It was further observed that average heat flux increases sharply with Reynolds number whereas the variation was not that steep with change in equivalence ratio. [Copyright &y& Elsevier]

Abstract: The heat and fluid flow characteristics of rectangular duct having its one broad wall heated and roughened with periodic ‘discrete V-down rib’ are experimentally investigated. Reynolds number (Re) has been varied from 3000–15000 with relative gap width (g/e) and relative gap position (d/w) range of 0.5–2.0 and 0.20–0.80 respectively. The respective variation in relative roughness pitch (P/e), angle of attack (α) and relative roughness height (e/Dh ) have been 4–12, 30°–75° and 0.015–0.043. The effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and the results obtained were compared with those of smooth duct. The maximum increase in Nu and f over that of smooth duct was 3.04 and 3.11 folds respectively. The rib parameters corresponding to maximum increase in Nu and f were d/w = 0.65, g/e = 1.0, P/e = 8.0, α = 60° and e/Dh  = 0.043. Correlations for the Nu and f in terms of Re and rib parameters have been developed. [Copyright &y& Elsevier]

Abstract: A combined experimental and numerical study has been conducted to investigate the occurrence of off-stagnation peak for laminar methane/air flame impinging on a flat surface. Experiments were conducted for three tube burners of internal diameter 8mm, 9.7mm and 12mm. Radial heat flux distributions were compared (experimentally) for different burner diameters under identical operating conditions (with firing rates of 0.25kW, 0.40kW and 0.50kW, ϕ =1 and H =40mm). An off-stagnation point peak in heat flux was observed for some of the configurations in the present study which is in accordance with the previous findings. This off-stagnation point peak is a function of stand-off distance between the exit plane of the burner and the plate and also the distance between flame-tip and the plate. A satisfactory explanation is presented to explain the existence of this off-stagnation peak with the help of results of numerical simulation carried out with commercial CFD code FLUENT. It is concluded that this off-stagnation peak in heat flux is primarily due to the peak in the axial velocity profile close to the impingement surface. [Copyright &y& Elsevier]

In this paper, results of experimental investigation on heat transfer and friction in rectangular ducts roughened with a new configuration of 'discrete V-down ribs' on one broad wall are presented. The discretization of V-down rib is done by providing small symmetrical gap equal to rib height in both legs of V-shape. The rectangular duct has width-to-height ratio of 12 and the Reynolds number ranged from 3000 to 15 000. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. The rib roughness has height-to-hydraulic diameter ratio of 0.043, angle of attack of 60°, and pitch-to-height ratio of 10. The relative gap position was varied from 0.20 to 0.80. The effects of gap position on Nusselt number and friction factor have been discussed and results compared with continuous V-down rib roughened duct and smooth duct under similar flow and boundary conditions to investigate the enhancement in Nusselt number and friction factor. The maximum enhancement in Nusselt number and friction factor has been obtained at relative gap position of 0.65 and is of order of 2.88 and 3.03 times of that of the smooth duct, respectively. The thermohydraulic performance of solar air heater provided with such rib roughness is considerably enhanced. [ABSTRACT FROM AUTHOR]

Abstract: A combined experimental and numerical study has been conducted to determine the stagnation point heat transfer for laminar methane/air flame impinging on a flat surface. Effects of Reynolds number, equivalence ratio and burner diameter on stagnation point heat flux were examined experimentally at different separation heights. Maximum stagnation point heat flux was obtained when the flat surface was closest to the tip of the inner premixed reaction zone. Heat flux decreased along the axial direction when the separation distance was further increased from the tip of inner reaction zone. There was a secondary rise in heat flux at the stagnation point at larger separation distances. Correlations were developed for stagnation point Nusselt number. Numerical simulations were carried out using a commercial CFD code (FLUENT) for laminar methane/air flame impinging on a flat surface for various separation distances. Results were compared with those found experimentally. The reason for conducting the simulations was to (a) gain more insight into how the presence of the plate affects the flame and the flow and temperature fields and (b) to explain the reason for high heat flux when the tip of the inner reaction zone was very close to the stagnation point. [Copyright &y& Elsevier]

Abstract: An experimental study has been conducted to determine the heat transfer characteristics of methane/air laminar flames impinging normal to a cylindrical surface. Effects of variations in the values of Reynolds number (Re =600–1300), equivalence ratio (ϕ =0.8–1.3), dimensionless separation distance (H/d =1–5), and burner diameter to cylinder diameter ratio (d/D =0.0538–0.1076) have been investigated. Three important configurations, viz., flame inner reaction zone far away, just touching and intercepted by the impingement surface, were examined in detail. High stagnation point heat fluxes were obtained when tip of the flame inner reaction zone just touched the target surface. Stagnation point heat fluxes were either zero or negative when the inner reaction zone was intercepted by the impingement surface. An off-stagnation peak in heat flux was obtained at moderate separation distances above the flame tip. Both stagnation point and peak heat fluxes increased with Re when the inner reaction zone length was less than the separation distance. Heat fluxes in the wall-jet region were high at high Re. Maximum heat fluxes were obtained for initially fuel-rich mixture conditions due to entrainment of the surrounding air. Smaller burner diameters produced high heat flux at the stagnation region for fixed Reynolds number and opposite trends were seen in the wall-jet region. A secondary rise in stagnation point heat flux was obtained at larger separation distances. This secondary rise in heat flux was quite significant for larger burner diameters and at low flow rates. Correlations were developed for stagnation point heat flux. Results were also compared with flat plate under identical operating conditions. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

An experimental study has been conducted to find the heat transfer characteristics of methane/air flames impinging normally to a flat surface using different burner geometries. The burners used were of nozzle, tube, and orifice type each with a diameter of 10 mm. Due to different exit velocity profiles, the flame structures were different in each case. Because of nearly flat velocity profile, the flame spread was more in case of orifice and nozzle burners as compared to tube burner. Effects of varying the value of Reynolds number (600–2500), equivalence ratio (0.8–1.5) and dimensionless separation distance (0.7–8) on heat transfer characteristics on the flat plate have been investigated for the tube burner. Different flame shapes were observed for different impingement conditions. It has been observed that the heat transfer characteristics were intimately related to flame shapes. Heat transfer characteristics were discussed for the cases when the flame inner reaction cone was far away, just touched, and was intercepted by the plate. Negative heat fluxes at the stagnation point were observed when the inner reaction cone was intercepted by the plate due to impingement of cool un-burnt mixture directly on the surface. Different heat transfer characteristics were observed for different burner geometries with similar operating conditions. In case of tube burner, the maximum heat flux is around the stagnation point and decay is faster in the radial direction. In case of nozzle and orifice burner, the heat transfer distribution is more uniform over the surface. [ABSTRACT FROM AUTHOR]

Abstract: A review of the literature pertaining to flame impingement heat transfer is presented. Studies related to different modes of heat transfer, flame shapes and flame stabilization are considered. Investigations of previous work for different experimental configurations, operating conditions, burner geometry, separation distance and stagnation target with instrumentation are compared. Studies related to measurement and contributions of non-luminous radiation to overall heat transfer are considered. Modeling and computation of flame impingement heat transfer processes in the recent literature have been reviewed. Semi-analytical and empirical correlations to find the total heat flux are briefly presented. Finally, unresolved issues are mentioned for further research. [Copyright &y& Elsevier]

The current experimental study is focused on examining the behaviour of heat transfer of pure R123 and R123/multi-walled carbon nanotubes nanofluid condensing in a smooth horizontal pipe at a range of different working conditions, like the mass velocity of about 200–500 kg/m2s, multi-walled carbon nanotubes volume fraction of approximately 0.01, 0.05, and 0.1 %, vapour quality of approximately 0.05–0.9, and saturation temperature of about 35.3 and 40.9 °C. The heat transfer coefficient of pure R123 and nanorefrigerant is compared in this investigation. According to the results, employing multi-walled carbon nanotubes improves the heat transfer coefficient for all selected volume fractions. It is observed that the heat transfer coefficient for both pure and nanorefrigerant falls as the saturation temperature rises from 35.3 to 40.9 °C. For a mass velocity of about 400 kg/m2s, 0.1 % fraction of multi-walled carbon nanotubes results in the highest heat transfer coefficient increase of 29 % compared to the base refrigerant. The mean absolute error analysis is evaluated to forecast the accuracy of the experiments by comparing the experimentally captured condensation heat transfer data with available correlations, which shows good agreement. A new correlation for flow condensation heat transfer coefficient is suggested, with an acceptable mean absolute percentage error of about 9.6 %. • An experimental rig is fabricated to study the flow condensation heat transfer of R123 using MWCNTs in a horizontal tube. • HTC is studied at 0.01-0.1 Vol% MWCNTs fractions for the mass flux of 200-500 kg/m2s and vapour quality of 0.05-0.9. • The enhancement in heat transfer coefficient of 29 % is achieved at 0.1 vol% MWCNTs for the mass flux of 400 kg/m2s. • Condensation heat transfer correlations from the literature are used to predict the reliability of the experimental setup. • A new correlation is proposed based on the present experimental data, which shows a good agreement with a MAPE of 9.6%. [ABSTRACT FROM AUTHOR]

Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k-e/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh=5, 7, and 9) and separation distance (H/Dh=3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance. [ABSTRACT FROM AUTHOR]

Flame jet impingement has been studied as an indispensable technique for direct heating applications suitable for metal processing industries. The current experimental and numerical investigation pursues to unfold the fluid dynamics and heat transfer characteristics for an obliquely impinging co-axial swirling flame jet. Co-axial flame used in the present work contains outer swirling flame together with an inner non-swirling flame. Steady state heat balance technique using one dimensional energy balance across the impingement plate has been employed to determine the local heat flux distribution. Due to obliquely impinging swirling jet, development of an asymmetric thermal boundary layer takes place on the target surface. The geometry being studied here develops a complex flow field which has also been investigated numerically using RANS based 'k-e' RNG turbulence model. The effect of various operating parameters such as inclination (θ = 0°, 15°, 30° and 45°), impingement distance (Z/D h = 3, 5, 7 and 9), Reynolds number (Re(s) = 2500, 4000, 5500, 7000 and 8500) and equivalence ratio (ϕ = 0.9, 1.0, 1.1 and 1.2) have been studied on the flow field development and resulting heat transfer distribution. • Obliquely impinging co-axial swirling flame jet with inner non-swirling flame. • Asymmetric heat flux patterns have been observed for inclined impingement plate. • Deflection of inner flame does not only depend on the plate inclination. • Deflection of inner flame is a stronger function of co-axial swirling flame behavior. • Structure of recirculation zone depends on inclination angle of plate. • Asymmetric recirculation lobes have been noticed for co-axial swirling flame. [ABSTRACT FROM AUTHOR]

Experimental and numerical study of linear arrays of co-axial flames impinging on a flat quartz plate have been performed and interactions and heat transfer characteristics are analyzed. High resolution inverse heat conduction procedure (IHCP) has been employed for determining local heat flux distribution. Strong interactions developed amidst adjacent flames led to asymmetric interactions. Combustion simulations revealed differences in the flow structures of four and five element arrays. Asymmetric interactions resulted in bending of all of the inner flames in four element array whereas central inner flame remains undeflected in five element array. Same has also been observed for six and seven element arrays too. Tilted columnar flows have been identified in even as well as odd numbered linear array of co-axial flames. These tilted flow structures resulted in distortion of recirculation zones (RCZ) of all the flames. Impingement heat flux contours are studied for various impingement spacings (H/D h = 3, 5 and 7). At smallest impingement spacing, these contour maps revealed footprints corresponding to central inner flame due to symmetrical interactions. Thermal efficiency and heat transfer uniformity of four element array has been observed to be higher than five element array at all impingement spacings. [ABSTRACT FROM AUTHOR]

In the present work, the thermal performance of impinging jet double pass solar air heater is investigated experimentally. A corrugated wavy plate consists of circular holes, each of 7.66 mm in diameter on the crest region are used to provide the impinging jets of air that provide 1.4%–0.48% plate perforation. While, the double pass is provided by circulating the air first through the channel formed between back and lower corrugated plates and then through another wavy channel formed between two corrugated plates. To provide thermal backup and increase heat transfer to air, two design configurations are investigated i.e. Design-I without porous media (100% porosity) and Design-II with porous media (95%–98% porosity) in the wavy channel. Moreover, the deep insight on physics of fluid using CFD tool is presented in the extent of this work. It is recommended to operate the present solar air heater designs at the mass flow rate of 0.04 kg/s, bed porosity of 98% and impinging plate perforation of 0.48% to have the best thermal performance. Whereas, the maximum thermal efficiency and thermohydraulic efficiency of 94% and 84% is obtained for Design-I, which is about 7.5% and 19%, respectively high compared to Design-II. • Two design of solar air heater with and without porous media are investigated. • Experimentally and numerically investigations are carried out. • Numerical results revealed the scope of thermal performance enhancement using jet impingement. • Thermohydraulic performance of Design-I without porous media is obtained high compared to Design-II. • Jet impingement using perforated metal wavy plate is attractive consideration to enhance thermal performance. [ABSTRACT FROM AUTHOR]

• Interactions result in clustering of high heat flux regions. • Flow circulation observed along the inner periphery of annulus. • Significant thermal dilution due to entrainment along the outer periphery of annulus. • Asymmetric interactions under impinging conditions deflected the inner flames. • Optimum performance observed at S/D h of 4 and H/D h of 6. Performance of gas burners can be optimized by improvements in their design and proper selection of operating parameters associated with combustion and heat transfer mechanisms. Array of multiple swirl burners is one such configuration where uniformity in heat transfer can be increased significantly. The current study presents experimental and numerical investigations conducted for flow patterns and impingement heat transfer developed in an annular arrangement of eight co-rotating dual swirling impinging flames. Impingement heat flux distribution has been studied experimentally using analytical inverse heat conduction procedure (IHCP). Effect of change in separation distance and inter-jet spacing has been studied for interactions and subsequent impingement heat transfer characteristics. Turbulent co-swirling flames portray intense mixing developed at the interaction regions due to strong interactions. Presence of impingement plate causes large scale changes in the flow-field. Numerical simulation conducted under reacting conditions predicted formation of asymmetric and distorted recirculation zones. Inner flames have been observed to deflect from their straight upright positions. Adjacent co-swirling flows merge together and develop circulation of flow at the inner periphery of the annulus. High heat fluxes observed at the target surface are corresponding to interaction regions and tend to cluster around the geometric centre of the array. Reversed downward flow developed at the geometric centre represents region devoid of heating. Averaged heat fluxes registered at the impingement plate decreased with increasing inter-jet spacing and separation distance. Maximum uniformity in the heat transfer has been observed at inter-jet spacing, S/D h of 4 and separation distance, H/D h of 6. [ABSTRACT FROM AUTHOR]