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Raymond Viskanta - One of the best experts on this subject based on the ideXlab platform.

  • effect of nozzle geometry on local convective heat transfer to a confined impinging Air jet
    Experimental Thermal and Fluid Science, 1996
    Co-Authors: D W Colucci, Raymond Viskanta
    Abstract:

    This article reports results on the effects of hyperbolic nozzle geometry on the local heat-transfer coefficients for confined impinging Air Jets. A thermochromatic liquid-crystal technique is used to visualize and record isotherms on a uniformly heated impingement surface. Experiments are conducted at low nozzle-to-plate spacings (0.25 < HD < 6.0) and Reynolds numbers in the range of 10,000 to 50,000 for two different confined, hyperbolic nozzles. As a reference, results have also been obtained for a confined orifice and are compared with those for the hyperbolic nozzles. The effects of Reynolds number, nozzle-to-plate spacing, and nozzle geometry on the local heat-transfer coefficients are reported and compared with similar experiments for unconfined Jets. It is concluded that the local heat-transfer coefficients for confined Jets are more sensitive to Reynolds number and nozzle-to-plate spacing than those for unconfined Jets.

  • convective heat transfer to a confined impinging array of Air Jets with spent Air exits
    Journal of Heat Transfer-transactions of The Asme, 1994
    Co-Authors: Aaron M. Huber, Raymond Viskanta
    Abstract:

    This investigation has examined the influence of spent Air exits located between the Jets on the magnitude and uniformity of the local heat transfer coefficient for a confined 3×3 square array of axisymmetric Air Jets impinging normally to a heated surface. The heat transfer coefficient was measured using a 0.025-mm-thick stainless steel impingement surface coated with liquid crystals. The temperature distribution along the surface was determined by measuring the reflected wavelength of light from the liquid crystal with the use of bandpass filters and an electronic digitizer board. The effect of small noise-to-plate spacings (0.25 and 1.0 diameters) commonly used in material processing applications was also considered

  • Effect of jet-jet spacing on convective heat transfer to confined, impinging arrays of axisymmetric Air Jets
    International Journal of Heat and Mass Transfer, 1994
    Co-Authors: Aaron M. Huber, Raymond Viskanta
    Abstract:

    Abstract The effects of jet-jet spacing ( X n D ), low nozzle-plate spacings ( H D = 0.25, 1.0 and 6.0 ) and spent Air exits located between the jet orifices were studied on the magnitude and uniformity of the convective heat transfer coefficients for confined 3 × 3 square arrays of isothermal axisymmetric Air Jets impinging normally to a heated surface. Local and average Nusselt numbers are presented for Reynolds number range of 3500–20 400. The local Nusselt numbers illustrate the (non)uniformity of the heat transfer and aid in understanding the variations in the average Nusselt number. The jet-jet spacing affects the convective coefficient by varying the influence of the adjacent jet interference and fraction of the impingement surface covered by the wall jet. The addition of spent Air exits increased the convective coefficient and influenced the location of the optimum separation distance. In addition, significant enhancement of the uniformity and the convective coefficients was observed at H D = 0.25 and 1.0 when compared to H D = 6.0 .

Y. Ozmen - One of the best experts on this subject based on the ideXlab platform.

  • confined impinging twin Air Jets at high reynolds numbers
    Experimental Thermal and Fluid Science, 2011
    Co-Authors: Y. Ozmen
    Abstract:

    Abstract An experimental study is carried out to investigate flow characteristics of confined twin Jets issuing from the lower surface and impinging normally on the upper surface. Pressure distributions on the impingement and confinement plates were obtained for Reynolds numbers ranging from 30,000 to 50,000, nozzle-to-plate spacing ( H / D ) in the range of 0.5–4 and jet-to-jet spacing ( L / D ) in the range of 0.5–2. Smoke-wire technique was used to visualize the flow behavior. The effects of Reynolds number, nozzle-to-plate spacing and jet-to-jet spacing on the flow structure are examined. The subatmospheric regions occur on both impingement and confinement plates at the nozzle-to-plate spacing up to 1 for all studied Reynolds numbers and jet-to-jet spacings in consideration. They lie nearly up to the same radial location at both surfaces and move radially outward from the stagnation points with increasing nozzle-to-plate spacing and jet-to-jet spacing. It is concluded that there exists a relation between the subatmospheric regions and peaks in heat transfer coefficients for low spacings in the impinging Jets.

  • An experimental investigation on flow structures of confined and unconfined impinging Air Jets
    Heat and Mass Transfer, 2005
    Co-Authors: E. Baydar, Y. Ozmen
    Abstract:

    The flow characteristics of both confined and unconfined Air Jets, impinging normally onto a flat plate have been experimentally investigated. The mean and turbulence velocities, and surface pressures were measured for Reynolds numbers ranging from 30,000 to 50,000 and the nozzle-to-plate spacings in range of 0.2–6. Smoke-wire technique is used to visualize the flow behavior. The effects of Reynolds number, nozzle-to-plate spacing and flow confinement on the flow structure are reported. In the case of confined jet, subatmospheric regions occur on both impingement and confinement surfaces at nozzle-to-plate spacings up to 2 for all Reynolds numbers in consideration and they lie up to nearly the same radial location at both surfaces. However, there is no evidence of the subatmospheric region in unconfined jet. It is concluded that there exists a linkage among the subatmospheric region, turbulence intensity and the peaks in heat transfer coefficients for low spacings in impinging Jets.

Jingai Hao - One of the best experts on this subject based on the ideXlab platform.

  • attrition of methanol to olefins catalyst with high velocity Air Jets at elevated temperature
    Advanced Powder Technology, 2015
    Co-Authors: Jingai Hao, Yinfeng Zhao, Zhongmin Liu
    Abstract:

    Abstract The Air Jets method proved to be an important tool for assessing attrition propensity of fluidized bed catalyst in the laboratory, but the influence of test temperature and test time has seldom been considered. This paper focuses on the influence of test temperature and test time on the attrition mechanism of methanol to olefins (MTO) catalyst in high velocity Air Jets. The attrition time was prolonged to 144 h in current study. It has been found that test time should be carefully chosen in order to achieve an equilibrium attrition rate. A short test time is not sufficient to understand the attrition mechanism. Test temperature affects the attrition mechanism significantly. At room temperature the abrasion and fragmentation were found coexisting while at 500 °C the abrasion is dominant. The abrasion of MTO catalyst at 500 °C results in a severe generation of superfine powder that is hard to capture by downstream cyclones, which is in accordance with the findings in industrial MTO unit. Apparently the attrition test at room temperature cannot reflect the generation of superfine powder. Both test time and test temperature should be carefully considered when the Air Jets method is used in the attrition study in laboratory.

Zhongmin Liu - One of the best experts on this subject based on the ideXlab platform.

  • attrition of methanol to olefins catalyst with high velocity Air Jets at elevated temperature
    Advanced Powder Technology, 2015
    Co-Authors: Jingai Hao, Yinfeng Zhao, Zhongmin Liu
    Abstract:

    Abstract The Air Jets method proved to be an important tool for assessing attrition propensity of fluidized bed catalyst in the laboratory, but the influence of test temperature and test time has seldom been considered. This paper focuses on the influence of test temperature and test time on the attrition mechanism of methanol to olefins (MTO) catalyst in high velocity Air Jets. The attrition time was prolonged to 144 h in current study. It has been found that test time should be carefully chosen in order to achieve an equilibrium attrition rate. A short test time is not sufficient to understand the attrition mechanism. Test temperature affects the attrition mechanism significantly. At room temperature the abrasion and fragmentation were found coexisting while at 500 °C the abrasion is dominant. The abrasion of MTO catalyst at 500 °C results in a severe generation of superfine powder that is hard to capture by downstream cyclones, which is in accordance with the findings in industrial MTO unit. Apparently the attrition test at room temperature cannot reflect the generation of superfine powder. Both test time and test temperature should be carefully considered when the Air Jets method is used in the attrition study in laboratory.

S V Prabhu - One of the best experts on this subject based on the ideXlab platform.

  • Air jet impingement technique for thermal characterisation of premixed methane Air impinging flame Jets
    Applied Thermal Engineering, 2016
    Co-Authors: Pramod Kuntikana, S V Prabhu
    Abstract:

    A major part of the domestic and the industrial thermal energy requirement for the heating purpose is achieved by the combustion of hydrocarbon fuels using burners. The present study reveals that thermal characterisation of impinging flame Jets can be performed with a steady state technique similar to thin metal foil technique used for impinging Air Jets. The target surface impinged by the premixed methane–Air flame jet is simultaneously cooled from the rear side by impinging Air Jets at different Reynolds number. One dimensional energy balance across the thickness of the plate is performed. The Nusselt number and the effectiveness distributions for a tube burner with the present technique matched reasonably well with the two-equation technique proposed in our previous work. Maximum deviations of 12% and 15% are observed for Nu and η respectively. Correlations are developed for the local Nusselt number and effectiveness in terms of flame jet parameters. The burner is analysed for the thermal efficiency. For premixed cone flames, it is observed that the thermal efficiency increases with the Reynolds number and equivalence ratio and decreases with the burner tip to plate spacing.

  • isothermal Air jet and premixed flame jet impingement heat transfer characterisation and comparison
    International Journal of Thermal Sciences, 2016
    Co-Authors: Pramod Kuntikana, S V Prabhu
    Abstract:

    Abstract The jet impingement heat transfer is a well-established technique for obtaining high heat transfer rates for many cooling and heating applications. Impingement heat transfer methods are widely used in domestic and industrial appliances. The impinging isothermal gas Jets are used for both heating and cooling. However, premixed flame Jets are used for heating the target surface. Present study is an attempt to compare the heat transfer characteristics of the isothermal Air Jets and premixed flame Jets. The isothermal Air jet having jet temperature of 30 °C (cold jet or ambient temperature jet), 100 °C (hot jet), and premixed methane–Air flame jet (stoichiometric mixture) with Reynolds numbers of 500, 750, 1000, 1250 and 1500 and nozzle or burner to plate spacings of 2 d , 4 d and 6 d are experimentally investigated for heat transfer characterisation. Thin metal foil technique is used for characterising isothermal Air Jets. For a fixed jet Reynolds number, the Nusselt number of isothermal Air jet is found independent of the temperature difference between jet and surrounding fluid. A steady state technique is proposed for characterisation of premixed flame Jets. The outcome of the present study reveals that the heat transfer characteristics of isothermal Air jet and the premixed flame jet are almost same. The higher thermal entrainment in premixed flame jet in comparison with isothermal Air jet results in a lower effectiveness. The present heat transfer data can be directly utilised for many practical applications.