Professor Arthur T. Johnson
Enhanced Heat Transfer, 2000, Vol. 7, pp. 289-310
Liangyou Tang
Michael M. Ohadi
Arthur T. Johnson
ABSTRACT
A study of single-phase convection and flow condensation heat transfer in horizontal copper tubes (8.81 mm inside diameter) was conducted using three refrigerants (HCFC-22, HFC-134a and HFC-410A). A smooth tube and three micro-fin tubes (axial, helical and crosshatch enhancement) were examined. Local-mean flow condensation data were experimentally obtained. Experimental conditions were selected to reflect typical operating conditions encountered in refrigeration and air-conditioning applications. All micro-fin tubes illustrated significant enhancement in single-phase convection and flow condensation. The cross-hatch enhancement performed particularly better in both single-phase convection and flow condensation. For the three refrigerants investigated, the refrigerant type seemed to have little influence on the enhancement mechanism of the micro-fin tubes examined. The experimental results are presented in Part I. Development of design equations is presented in Part II.
Enhanced Heat Transfer, 2000, Vol. 7, pp. 311-325
Liangyou Tang
Michael M. Ohadi
Arthur T. Johnson
ABSTRACT
An empirical study of single-phase convection and flow condensation heat transfer in horizontal tubes was conducted. Three refrigerants (HCFC-22, HFC-134a and HFC-410A), a smooth tube, and three micro-fin tubes (axial, helical and crosshatch enhancement) were examined. Commonly cited correlations were evaluated, utilizing the experimental data obtained in Part I of this study. Although these correlations had fairly good agreement with HCFC-22 and HFC-134 results, all of them failed to predict HFC-410A performance. A modified Shah equation was developed for smooth tube annular flow condensation, which overcomes the shortcomings of the existing correlations. Furthermore, design equations were developed for single-phase heat transfer and two-phase condensation with all three micro-fin tubes investigated, and covering all three refrigerants tested.
J. Agric. Engng Res. (1992) 51, 15-27
Liangyou Tang
Arthur T. Johnson
ABSTRACT
Mixed convection conditions may exist when cooling field-fresh fruits. Experiments to determine convection coefficients were performed on cast models of apple, peach, plum, and strawberry shapes. Mixed convection Nusselt numbers were found to be obtainable from those for pure forced and natural convection.
J. Agric. Engng Res. (1991) 50, 197-208
L. Tang
A. T. Johnson
R. H. McCuen
ABSTRACT
This paper presents a study of the relation between mixed convection about a sphere and the extremes of forced and natural convection. Previous studies have shown that the opposing flow region must be treated separately from the cross and aiding flow region. We show that the Effective Diameter Scalar Addition method can be used to predict mixed convection in the latter and a composite power curve can be used to predict opposing flow mixed convection. Fundamental understanding gained from the sphere will be used in subsequent work applied to realistic shapes of fruits and vegetables.
1990 American Society of Agricultural Engineers, Vol. 33(1):January-February
S. Sirivicha
A. T. Johnson
L. W. Douglass
A. Kramer
ABSTRACT
Potato plugs 4, 8, and 12 mm thick were tested for gas diffusion of CO, CO2, and SO2. Temperatures of 10, 20, and 30o C and across sample pressure differences of -5.76, -56.6, and -106 kPa were used to determine diffusion coefficient value variation. Each sample was inserted into a custom diffusion chamber to study gas transport and the transient Fick equation for porous materials was used to analyze the data. Values for diffusion coefficients for all gases were found to be nearly identical at about 2 x 105 cm2/s. Porosity of the potato tissue was measured to be about 0.15, and tortuosities were 0.8, 0.05, and 0.005 for CO, CO2, and SO2, respectively. Sample thickness was found to be a much greater influence on diffusion coefficient values than either temperature or pressure.
Int. Comm. Heat Mass Transfer, Vol. 17, pp. 67-77, 1990
Liangyou Tang
Arthur T. Johnson
ABSTRACT
Mixed forced and natural convection from spheres has been postulated to depend on an effective sphere diameter, which is determined by the angle between forced and natural flows [1,2]. This hypothesis was presented without direct visual evidence to support it. Smoke flow pattern photographs show that aiding flow has a weaker natural component than cross flow, and that there are various patterns during counter flow to form a heat transfer minimum and maximum. This photographic study confirms the hypothetical basis upon which combined forced and natural convection is determined from each mode separately.
1988 American Society of Agricultural Engineers, Vol. 31(1):January-February, 1988
Arthur T. Johnson
G. Douglas Kirk
Soung H. Moon
Tien-Mo Shih
ABSTRACT
Mixed convection heat loss from spheres was studied using both experimental and numerical technique. Results showed good agreement between approaches. The different techniques are each shown to yield valuable information concerning mixed convection. Results can be used to better understand heat transfer from such agricultural products as fruits and vegetables.
Numerical Heat Transfer, vol. 13, pp. 65-89, 1988
S. H. Moon
T. M. Shih
A. T. Johnson
ABSTRACT
Combined forced and free convection around horizontal cylinders as well as spheres has been numerically investigated, with the emphasis placed on the pressure solutions. The stream function-vorticity formulation is adopted in conjunction with hybrid (central and upwind) difference and nonuniform grids. For recovery of the pressure distribution, which seems unavailable in the literature, the pressure Poisson's equation is solved after convergence of the stream function and vorticity solutions has been achieved. Computed results of Nusselt numbers are compared with several reported literature values. Good agreement has been found between their values.
J. Agric. Engng Res. (1987) 38, 289-300
S. H. Moon
A. T. Johnson
T. M. Shih
ABSTRACT
Mixed convective heat transfer from horizontal cylinders was investigated using numerical procedures. Elliptic partial differential equations were transformed into finite difference equations and numerical results were computed using a non-uniform grid. A method to reliably verify the procedure is given, along with a comparison between results from this study and prior work. The results of this analysis agree to within 5% with published experimental work. Increasing flow rate over the object will not necessarily increase heat transfer by mixed convection, because of flow separation effects. The results of this study may be applied in the post-harvest cooling of agricultural produce of relevant shape.
Int.. Comm. Heat Mass Transfer, Vol. 13, pp. 369-387, 1986
G. Douglas Kirk
Arthur T. Johnson
ABSTRACT
Experimental data for mixed convection heat loss from spheres was collected over a range of angles between forced and natural convection flows. The results indicate that opposing flow must be treated differently from cross and aiding flows. A hypothetical explanation is given for the general results.
Energy Vol. 6, No. 9, pp. 895-899, 1981
Printed in Great Britain
S. H. Moon
K. E. Felton
A. T. Johnson
ABSTRACT
Optimum tilt angles for solar collectors at 171 locations in North America were determined. As a matter of user convenience, equations predicting optimum collector tilt angle were derived by using regression analysis based on the results obtained for the 171 locations for several different periods of the year. These equations adequately predict the optimum tilt angles as indicated by the R2 values of 0.814 to 0.920.
Transactions of the ASAE, Vol. 24, No. 2, Mar.-Apr. 1981
Arthur T. Johnson
G. Douglas Kirk
ABSTRACT
Botsball and WBGT sensors are two ways currently used to measure environmental heat stress. Thermodynamic and heat transfer relations are developed for these sensors in order to obtain basic understanding of the way these two measurements are related. Slight modifications of our equations were accomplished to match published natural wet bulb and Botsball data. An analysis such as this enables standard WBGT information to be determined from nonstandard, but more convenient, means of measurement.