Heat exchanger design: Optimal thickness (under natural convective conditions) of vertical rectangular fins protruding upwards from a horizontal rectangular base

Steady-state rates of heat transfer from an array of vertical, rectangular polished duralumin fins under natural convective conditions have been measured. The horizontal base, which was manufactured of the same material, was kept at the uniform temperature of either 20·0 (±0·2)°C or 40·0 (±0·2)°C above the local air temperature of 20 (±0·2)°C.

The optimal thickness of the fins in this array, corresponding to a maximum rate of heat dissipation, was deduced. For a base of width 190 mm and length 500 mm, the optimal thickness for fins of 60 mm protrusion rose from 2·0 mm to 4·5 mm when the fin separation was increased from 20 mm to 60 mm. This optimal fin thickness was almost invariant with respect to the change of the considered base temperature.     

An experimental study of centrifugally driven free convection in a rectangular cavity

Centrifugally driven thermal convection has been studied experimentally in a rectangular cavity of length 27·94 cm, height 2·62 cm, and width 2·54 cm. The cavity was heated from above and cooled from below and rotated about a vertical axis passing through the center point of the cavity. The cavity was filled with silicone oils having Prandtl numbers of 7 and 3000, and rotated up to 565 rev/min.

The Nusselt number for heat transfer from the top to bottom surfaces varied as the 0·4 power and the 0·25 power of the imposed temperature difference for the low and high Prandtl number fluids, respectively, and as the 0·25 and 0·3 powers of the centrifugal acceleration evaluated at the outer edge of the cavity. A dimensionless radial centerline temperature gradient was obtained and found to be independent of both rotational rate and imposed temperature difference.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in the ground; the ground is covered with a glazed thermal trap. The heat is extracted by means of a flow of liquid in such a way that the collection temperature remains constant. An expression has been derived for the periodic rate at which heat can be retrieved to keep the collection temperature constant. Numerical calculations for a typical cold day in New Delhi predict that, for a collection temperature of 30°C and for a trap thickness of 0·03 m, the maximum possible integrated collection efficiency is 25·7 per cent.     

Digital pyrometry in a solar furnace

The brightness temperature and emissivity measurements of an irradiated specimen at the focal point of a solar furnace have been performed by the use of a brightness pyrometer to which an electrical recorder, an oscilloscope and Polaroid camera were attached previously. The use of such a photographic method gave reading errors as ±2K at 3000°C and ±15K at 2000°C.

Therefore, a digital computer system has been coupled with the brightness pyrometer in order to give the higher accuracy in brightness and true temperature measurements and to process a large amount of data. The system consists of A/D converter, central processing unit, graphic display and other transfer units. After an analog signal from the brightness pyrometer was amplified by a dc amplifier, the signal converted into digital form was operated by the digital computer. Conversion errors in the data processing were found to be ±1K at 3000°C and ±7K at 2000°C respectively. The freezing point data by the new system on refractory oxides such as ZrO₂ and Y₂O₃ were presented.     

Thermal design of a roof as an inexpensive solar collector/storage system

This paper presents the thermal performance of a roof as a solar collector/storage system which is important for the thermal design of buildings. The system consists of a mass of concrete or concrete insulation, one face of which is blackened/glazed and exposed to solar radiation and ambient air, while the other is in contact with room air at constant temperature. The heat can be extracted by the passage of water through the network of tubes in this block. It is seen that, by increasing the depth of the tubes, the rise in water temperature decreases but the time difference between the maxima of the solair temperature and that of the outlet water temperature increases. At a tube depth of 0·10 m, the maximum temperature rise of the water is 33·5°C. The corresponding efficiency of the system is 28·0% while the flow rate of water is 5·0 litre/h m²; the heat flux entering the room is also reduced considerably.     

Creep deformation of induction pressure welded 2·25Cr-1Mo steel

In this paper, experimental results involving the effect of stress and temperature on creep behaviour of induction pressure welded (IPW) 2·25Cr-1Mo steel are presented. Creep rupture tests were conducted at 550–700°C in steps of 50°C over a stress range of 112·5–180 MPa. Above 650°C failure of the specimen was enhanced due to the microstructural instability. Failure in the specimens occurred invariably in the heat affected zones (HAZ), and the fracture surfaces indicated ductile failure.     

Report of the first prototype of non-imaging focusing heliostat and its application in high temperature solar furnace

Following the publication on the principle and theory of a newly proposed non-imaging focusing heliostat, this paper presents a report on the design, optical alignment and application of the first prototype heliostat. In the architecture of the first prototype, 25 mirrors, each with a dimension of 40×40 cm, are arranged into five rows and five columns to form a total reflective area of 4 m². The design of the essential part of the first prototype heliostat will be discussed in this paper, which consists of two primary elements; a rotation–elevation system for tracking a mirror support frame which carries 25 mirror facets, and a separate two-axis tracking system for compensating (each second) off-axis aberrations of 24 slave facets relative to the central mirror, which is fixed in the mirror frame. The rotation–elevation system consists of a pedestal supporting a rotational tracking mechanism carrying a U-shaped arm and a second tracking system for tracking a moving frame in elevation. The moving frame carries a central stationary (relative to the frame) mirror, called a master mirror. Slave mirrors are arrayed in five rows and five columns, and eight stepper motors drive the outer four rows and columns relative to the master mirror via a computer programme implementing newly proposed formulas to eliminate the first-order aberration. With a second stage concentrator comprising a small aperture size parabolic mirror (diameter of 60 cm), a cost-effective high temperature solar furnace was constructed. In our experiment, the highest furnace temperature of 3400°C has been recorded through the melting of pure tungsten wires.     

Chemical vapour deposited SnO2:Sb heat mirror coatings for cylindrical solar collectors

A chemical vapour deposition technique for growth of SnO₂:Sb heat mirror coatings on the inner walls of long cover glass tubes for cylidrical solar collectors is reported. The best performance of the tin-oxide film is obtained for those films grown from a source of SnCl₂ + 1 mol% Sb on Corning glass tubes at 520°C. These films, supported on 2 mm glass substrates, have a solar transmittance of 0.85 and an infrared reflectance of 0.8. The heat mirror coatings are observed to increase the stagnation temperature of the absorber in anevacuated tubular collector from 142 to 161°C under incident optical flux of 1150 W/m²     

Design and performance study of a solar energy powered vaccine cabinet

An insulated steel sheet cabinet of 0.6 × 0.3 m face area and 0.5 m depth was designed. The cabinet was intended to store vaccine in remote desert areas, away from the electrical national grid. World Health Organization regulations limit the inside temperature of such vaccine storage cabinets to the range of 0–8°C. A solar energy powered absorption refrigeration cycle using Aqua-Ammonia solution was designed to keep this cabinet temperature in the range of required temperatures, away from the outside temperature, which reaches about 45°C in August. A computer simulation procedure was developed to study the performance and characteristics of the cooling cycle. The simulation included MATLAB computer programs for calculating the absorption cycle, thermodynamic properties and quantities as subroutines for the main program, and a detailed mathematical model using EXCEL for the solar side of the unit. A year round work was predicted. Refrigeration cycle coefficient of performance ranged between 0.5 and 0.65; cylindrical solar concentrator extended the daily operating time to about 7 h and increased the output temperature up to >200°C, while the temperature which gives optimum condition (of COP = 0.65) was 120°C.     

Natural-convection characteristics of a horizontally-based vertical rectangular fin-array in the presence of a shroud

The steady-state natural convective cooling of horizontally-based, vertical rectangular fins, when in close proximity to an adiabatic horizontal shroud, situated adjacent to and above the horizontal fin-tips, was investigated experimentally. The resuls are of significance for the designers of electronic arrays, the components of which should be maintained at temperatures less than 65°C in order to ensure operational reliability. The optimal fin separation, which corresponds with the maximum rate of heat loss from the fin-array, has been deduced for various combinations of fin protrusions and distances of the shroud above the vertical fins, when the fins' base was maintained at a uniform temperature of 40 ± 0·5°C above that of the environment (21·0 ± 0·5°C). For a constant temperature-difference between the fin-base and the environment, lower optimal fin separations and higher steady-state heat-dissipation rates ensued when the shroud clearance to the fin height ratio was increased from zero to unity. Increasing the fin protrusion above the horizontal base also resulted in higher heat-dissipation rates from the fin-array. However, the fin-array with maximum shroud clearance is a much more favourable configuration (e.g. with respect to requiring less material) for achieving heat-transfer enhancement, than the fin array which employs large fin protrusions. For an open-ended duct of approximately the dimensions considered in this project (i.e. of maximum rectangular section 240 mm × 180 mm), the fins should protrude to less than half the internal height of the duct in order that a high convective performance of the fins is achieved. There is an optimal value of the shroud clearance to fin height ratio which exceeds unity for each specific operation, i.e. the exact optimal ratio being dependent upon the geometry and temperatures involved.

Average Nusselt numbers, evaluated from the experimental data, are correlated non-dimensionally with respect to the fin-array geometry and the Grashof number. This correlation indicates that variations of the shroud clearance to fin height ratio produce only marginal variations in the average Nusselt number.     

Transient boiling of liquefied cryogens on a water surface: II. Light hydrocarbon mixtures

Mixtures of light hydrocarbons characteristic of liquefied natural gas were boiled on a water surface and the rate of vaporization measured. Heat fluxes were significantly higher than measured for pure liquid methane even when mole fractions ethane, propane, or n-butane were under 0·01. As with pure methane, the rate of vaporization increased during the course of an experiment unless a continuous, thick ice layer formed. Initial liquid water temperatures ranged from 6 to 60°C and spills of LNG from 0·21 to 0·83 g/cm₂ were made. For mixtures of methane and heavier hydrocarbons, where the mole percent of the latter components was greater than 2 per cent, the water surface temperature rapidly dropped to the cryogen temperature, but little change in temperature was noted a few mm below the surface. No significant vapor superheat was noted. Ice formed readily at the interface and the increase in heat flux was explained by postulating a shift from film to nucleate boiling. The boiling mixtures often foamed and photographs taken at the interface revealed that very small vapor bubbles formed early in a run.     

Concentration of the solar radiation in a solar furnace

Energy concentration in a solar furnace is greatly influenced by the optical accuracy of the reflecting surface of the mirror. The highest density of a heat flux on the heating surface of a specimen is determined by its concentration and by the reflectivity of the mirror.

Since the apparent diameter of the moon is almost equal to that of the sun and the illumination of the moon may be adapted to the photographic determination of flux density, the rate of energy concentration in the authors' solar furnace was studied by the moon's image projected upon the heating zone in place of that of the sun.

As a result, about 300 watts per cm was obtained as the highest possible density of the heat flux in this solar furnace. This figure also agreed with the results of an optical analysis based on the practical finishing of the present mirror surface. With a heat flux of 300 watts per sq cm, in accordance with Stefan-Boltzmann's law of radiation, the highest temperature attainable is estimated to be approximately 2700°K. Besides the above, in actual experiment, approximately 2300°C was obtained in observing the melting points of binary mixtures, e.g., MgO---CoO and MgO---Cr₂O₃. Thus, the above-mentioned three approaches to the problem are in fairly good agreement.     

Heat transfer performances of vertical rectangular fins protruding from rectangular bases: Effect of fin length

The effects of increasing the fin length from 250 to 375 mm on (i) the steady-state rate of heat loss and (ii) the optimal uniform fin separation of vertical rectangular fins protruding from a horizontal or a vertical rectangular base, have been investigated experimentally. A constant base temperature, 40 (±0·3)°C above that of the ambient environment, was used.     

Experimental study of gas humidification with injectors for automotive PEM fuel cell systems

A scaled gas humidification system using injectors for PEM fuel cell vehicles was developed and the humidification performance was evaluated under various operating conditions. The humidification system consists of an injector, a duplex enthalpy mixer and a water management apparatus. A dew point meter of the chilled mirror type was used to measure the humidity of the air and the hydrogen. Humidification performance was evaluated by measuring the dew point temperature of the humidified gases. Humidification performance was observed to be critically affected by the temperature of injected water and the gas flow rate in this study. The dew point of the humidified gas rose when the temperature of injected water increased, however, it dropped when the gas flow rate was increased. Experimental results show that the outlet temperature was 58.4 °C, dew point temperature of the humidified air reached 54.0 °C when the injection water temperature was 69.5 °C with the room temperature air flow rate of 200 L min⁻¹. Inlet gas temperature also affected the humidification performance and response time. In addition, a 50 cm² PEM fuel cell was tested to verify the effectiveness of the devised humidifier. When operated at 65 °C, the fuel cell showed an operating voltage of 0.5 V at a current density of 600 mA cm⁻².     

Forced steady-state convections from pin-fin arrays

The steady-state thermal and air-flow resistance performances of horizontally-based pin-fin assemblies have been investigated experimentally. The effects of varying the geometrical configurations of the pin-fins and the air-flow rates have been studied. The optimal pin-fin separation S_xopt in the span-wise direction, to achieve a maximum rate of heat transfer from the assembly, is 1·0 ± 0·2 mm ≤ S_x ≤ 3·0 ± 0·2 mm for the pin-fins arranged either in the in-line or the staggered array. The optimal pin-fin separations in the stream-wise direction for these two arrays are 7·6 ± 0·2 mm and 7·8 ± 0·2 mm respectively. The general correlation of the data is:

     

Properties of spray-deposited cobalt oxide selective coatings on aluminium and galvanised iron substrates

Highly stable selective coatings of cobalt oxide have been prepared on commercial aluminium and galvanised iron substrates by the method of spray pyrolysis. The optothermal, structural and optical properties of these films have been investigated to determine the optimum growth parameters. Best selectivity values are obtained for films with thicknesses 0·30 μm on aluminium and 0·32 μm on galvanised iron. The optimised films on aluminium give = 0·88 and _100°C = 0·15 whereas those on galvanised iron give = 0·88 and _100°C = 0·13. The films are extremely adherent and are stable up to 500°C in the case of aluminium and 350°C in the case of galvanised iron substrates.     

Effects of temperature on the cyclic deformation behaviour and microstructural changes of 12Cr-1MoVW martensitic stainless steel

Strain-controlled uniaxial push-pull fatigue tests were carried out with cylindrical specimens of 12Cr-1MoVW steel from room temperature up to 600°C. Specimens were tested at a total strain range of 2% at room temperature, 300°C, 400°C, 500°C and 600°C, respectively, and were also tested at 500°C for a total strain range of 1·6%, 1% and 0·6%. The fatigue test results showed that the cyclic deformation behaviour of this steel was strongly dependent on the test temperatures. TEM studies revealed that cellular dislocation structure comprised the major microstructure and the subgrain size increased with increasing test temperatures. Carbides tended to redistribute along the subgrain boundaries in the specimens subjected to low cycle fatigue at high temperatures. The undeformed section of each specimen exhibited a completely different microstructure from that in the gauge section in all the temperatures tested. The lath martensite structures were not changed obviously when HT-9 steel cyclic deformed at lower strain range.     

Preliminary irradiation test results from the Yankee Atomic Electric Company reactor vessel test irradiation program

The Yankee Atomic Electric Company test irradiation program was implemented to characterize the irradiation response of representative Yankee Rowe reactor vessel beltline plate materials and to remove uncertainties in the analysis of existing irradiation data on the Yankee Rowe reactor vessel steel. Plate materials each containing 0·24 w/o copper, but different nickel contents at 0·63 w/o and 0·19 w/o, were heat treated to simulate the Yankee vessel heat treatment (austenitized at 982°C (1800°F)) and to simulate Regulatory Guide 1·99 database materials (austenitized at 871°C (1600°F)). These heat treatments produced different microstructures so the effect of microstructure on irradiation damage sensitivity could be tested. Because the nickel content of the test plates varied and the copper level was constant, the effect of nickel on irradiation embrittlement was also tested. Correlation monitor material, HSST-02, was included in the program to benchmark the Ford Nuclear Reactor (University of Michigan Test Reactor) which had never been used before for this type of irradiation program. Materials taken from plate surface locations (versus 1/4T) were included to test whether or not the improved toughness properties of the plate surface layer, resulting from the rapid quench, are maintained after irradiation. If the improved properties are maintained, pressurized thermal shock calculations could utilize this margin. Finally, for one experiment, irradiations were conducted at two irradiation temperatures (260°C and 288°C) to determine the effect of irradiation temperature on embrittlement. The preliminary results of the irradiation program show an average temperature effect of 38°C for a 28°C difference in irradiation temperature. The results suggest that for nickel bearing steels, the superior toughness of plate surface material is maintained after irradiation and for the copper content tested, nickel has little effect on irradiation response. No apparent microstructure effect on irradiation response was noted and the HSST-02 material's response to irradiation was similar to results from power reactor and other test reactor tests, thus qualifying the Ford Test Reactor for irradiation experiments such as those conducted for the Yankee Atomic program.     

Heat exchanger design: Optimal uniform separation between rectangular fins protruding from a vertical rectangular base

Steady-state rates of heat loss, from an array of 3 mm thick, 250 mm long, horizontal rectangular duralumin fins extending 60 mm perpendicularly out of a 250 mm × 190 mm vertical rectangular duralumin base, have been measured. With the base, at a uniform temperature of between 40°C and 80°C, in a 20°C ambient environment, two separations of the parallel fins, corresponding to maxima in the rate of heat loss versus fin separation curves ensued, respectively at 12 ± 1 mm and 38 ± 1 mm. The use of the latter maxima (i.e. the optimal separation) leads to the higher rate of heat loss. The heat transfer performances of vertical and horizontal rectangular fin arrays on a vertical rectangular base are compared: using the same geometrical configuration and identical base temperatures in both cases, the vertical fin orientation has the more rapid, steady-state heat loss.     

Design and performance characteristics of a parabolic-trough solar-collector system

A comparison of the advantages and disadvantages of concentrating collectors against conventional flat-plate collectors are presented. This is followed by the design of a parabolic-trough solar-collector system, due consideration having been given to collector-aperture and rim-angle optimisation, together with the receiver-diameter selection. The collector characteristic curve gives a test slope of 0·441 and a test intercept equal to 0·642. The value of the test slope differs considerably from the initially predicted value: this is attributed to the heat losses from the receiver support brackets. Subsequent allowance for these losses is presented: this reduces the difference from 24·9% to 5·7%. Other tests are presented, including the determination of the collector's incidence-angle modifier, time constant and acceptance angle.