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.     

Conversion of rice husk into amorphous silica and combustible gas

Results of the studies on thermogravimetric analysis (TGA) of raw and 1(N) HCl acid treated rice husk (in still air) reveal that its thermal degradation takes place in three main stages of mass loss, namely (i) drying, (ii) devolatilization and (iii) slow oxidation of fixed carbon. Hydrochloric acid leaching of husk at 75° for 1 h prior to combustion is necessary for production of amorphous silica of complete white colour. For production of low calorie-combustible gas along with amorphous silica from the rice husks containing 5.5–7% (wb) moisture, a furnace set temperature of 450°C appears to be optimal.     

Some aspects of fluidized bed combustion of paddy husk

Some agricultural wastes—for example, bagasse, paddy husk, etc.—are frequently used as fuels, paddy husk appearing, in particular, to be quite suitable for fluidized bed combustion. The conventional method of combustion of paddy husk in grate-type furnaces is slow and inefficient.

This paper reports certain aspects of the fluidized bed combustion of paddy husk. Fluidized bed combustion was carried out by feeding husk in a bed of sand particles. The unexpanded bed height was 10 cm and the size of the sand particles, 351–420 μm. The superficial velocity of the ambient fluidization air through the bed ranged from 11·1 to 22·2 m/min.

A combustion intensity of about 530 kg/h/m² of distributor area could be achieved. This is about 7·5 times higher than the maximum combustion intensity possible in a grate-type furnace per unit grate area. The efficiency of combustion, which ranged from 81 to 98 per cent was found to increase with the air flow rate. There was significant carry-over of inert sand particles from the bed under conditions of high air flow rate.

Combustion intensity increased as the bed height rose from 10 cm to 15 cm, but increased sand entrainment also occurred.     

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.     

Fatigue life response of ASME SA 106-B steel in pressurized water reactor environments

Fatigue strain-life tests were conducted on ASME SA 106-B piping steel base metal and weld metal specimens in 288°C (550°F) pressurized water reactor (PWR) environments as a function of strain amplitude, strain ratio, notch acuity, and cyclic frequency. Notched base metal specimens tested at 0·017 Hz in 0·001 part per million (ppm) dissolved oxygen environments nearly completely used up the margins of safety of 2 on stress and 20 on cycles incorporated into the ASME Section III design curve for carbon steels. Tests conducted with smooth base metal and weld metal specimens at 1·0 Hz showed virtually no degradation in cycles to failure when compared to 288°C air test results. In all cases, however, the effect of temperature alone reduced the margin of safety offered by the design curve in the low cycle regime for the test specimens. Comparison between the fatigue life results of smooth and notched specimens suggests that fatigue crack initiation is not significantly affected by 0·001 ppm dissolved oxygen, and that most of the observed degradation may be attributed to crack growth acceleration. These results suggest that the ASME Section III methodology should be reviewed, taking into account the PWR environment variables which degrade fatigue life of pressure-retaining components.     

Natural convection in an inclined rectangular channel at various aspect ratios and angles—experimental measurements

Rates of heat transfer were measured for laminar natural convection in silicone oil and air in a long rectangular channel. The aspect ratio (width/height) of the cross-section of the channel was varied over 1, 2, 3, 4·2, 8·4 and 15·5, and the Rayleigh number from 3 × 10³ to 10⁵. The channel was heated from below and cooled from above while the other two sides were insulated. The channel was then rotated about the long axis in steps through 180 degrees. The effect of inclination and of the aspect ratio on the rate of heat transfer was measured experimentally.A minimum and a maximum rate of heat transfer occurred as the angle of inclination was increased from 0 to 180 degrees. The angle of inclination at these critical conditions was found to be a strong function of the aspect ratio and a weak function of the Rayleigh number. A transition in the mode of circulation occurred at the angle corresponding to the minimum rate of heat transfer.     

Heat losses from a salt-gradient solar pond

A theoretical approach for the calculation of heat losses from a cylindrical flat-bottomed salt-gradient solar pond is discussed. Steady-state heat losses from the sides and bottom of the pond have been estimated, when the pond is uninsulated as well as insulated. The insulating materials considered in the present study are dry sand, mud powder, dry cement, marble dust and mica powder. The effective insulation is varied by varying the thickness and interstitial air pressure of the insulating materials. We find that the losses are reduced to a minimum with a marble dust wall of thickness 0·20 m at an interstitial air pressure of 0·5 mm of mercury.     

Experimental optimization of activated carbon synthesis by the simplex search method

To compete with the low price of petrol, thermochemical conversion of biomass should be viewed as both an energy and chemical source. Catalytic pyrolysis can produce activated carbon as a chemical and energy as a by-product, if inexpensive catalysts are available. In this paper, two parameters, namely NaCl (used as catalyst) and temperature, were found to affect the quality of the activated carbon produced from coconut shell and sawdust. Without mathematical modelling of the process, the Simplex search method led to the following maximum iodine numbers: for coconut shell, 685·8 mg/g at 794°C and 3·15 g NaCl/g coconut shell; for sawdust, 581·1 mg/g at 751°C and 5·52 g NaCl/g sawdust.     

A seasonally adjusted concentrator with modifications of absorber shape

The heat losses from concentrating solar collectors can be reduced by thermally isolating the absorber with an optimal air gap between the absorber and the glass cover. However, in reflectors attempting to attain maximum optical concentration, some of the reflected rays will escape through this gap, resulting in increased losses. A concentrator was fabricated and tested with a plane tubular absorber, the results having been reported earlier. The present paper describes a design procedure for an absorber which avoids the ‘optical’ losses through the air gap. The results for the modified absorber are compared with those for the plane tubular one. The intercept factor is improved from 0·8 to 0·92. The surface area of the modified absorber is about 9 % larger than that of the plane tubular one, but the shape is such that the overall heat loss factor is much lower. As a result, the total heat loss for the modified absorber at a temperature 100° C above ambient is only 0·2 % higher than that of the plane tubular one. The improved performance curves are shown. The absorber design is also acceptable from the point of view of strengths of the components involved.     

Evaluation of the performance of air heaters of conventional designs

Air heaters, because of their simplicity, are more amenable to theoretical investigations than the other solar thermal systems like solar stills, etc. In air-heating collections various parameters on which its performance depends can be identified and their effect can be studied theoretically. In this paper the transient equations of three types of air heater are written and their solutions are attempted. In Type I air heater there is one air channel above a metallic plate, in Type II, the air channel exists between two metallic plates and in Type III, there are two air channels one above the plate and another between two metallic plates. The transient equations of the model are solved within the framework of periodic analysis. The temperature of any component of the air heater is represented by a Fourier series in time, the coefficients being coordinate dependent. The effect of flow rate, plate length, selective coating, etc. are studied. It is shown that for a given rate of flow, there is an optimum value of plate length for which efficiency is maximum. The optimum value of plate length in Type II air heater increases as rate of flow increases. For a given plate length, the efficiency increases with rate of flow. The selective coating will be useful only when plate length is large and rate of flow is small. For a flow rate of 100 kg m⁻²hr⁻¹ and plate length of 7 m, the difference between outlet air temperature from a selectively-coated plate and that of black-painted plate, at midday, is 33°C. The selectively-coated air heater is 21 per cent more efficient than the black pained one at the same condition. It is found that an efficient configuration has lower glazing temperature.     

Comparative study of creep behaviour of 2·25Cr-1Mo steel and its weldments

Creep rupture tests under uniaxial loading condition were performed in atmospheric air on 2·25Cr-1Mo steel and its welded joints, namely Induction Pressure Welded (IPW) and Manual Metal Arc Welded (MMAW) joints, at test temperatures of 550 and 600°C and stresses of 110–180 MPa. The data obtained were analysed and the creep behaviour of unwelded material (UW) and the welded joint were compared. Under the given conditions, the IPW joints performed better than the base metal, but the MMAW joints showed less resistance to creep than the unwelded 2·25Cr-1Mo steel.     

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.     

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:

     

Energy-use patterns under various farming systems in Punjab

The state of Punjab (India) is divided into six agro-climatic zones, viz, sub-mountainous undulating zone, undulating zone, undulating plain zone, central plain zone, western zone and flood plain zone. Because of the associated vast changes in agro-climatic conditions, the distribution of energy consumption for crop production has not been uniform. For this study, the state was divided into four farming systems depending upon the level of energy use in crop production, irrigation facilities available and the status of farm machinery use. The four systems were: (1) traditional (rain-fed); (2) improved traditional (partially irrigated); (3) semi-intensive (irrigated) and (4) intensive (irrigated using improved farm implements). Multi-stage stratified techniques were applied in conducting the farmers' survey to study the energy-use pattern under various farming systems for the cultivation of wheat. The grain yield was a minimum (612 kg/ha) for farming system (1) and was a maximum (4677 kg/ha) for farming system (4). The specific energy requirement decreased from 8·84 MJ/kg (for farming system (1)) to 4·14 MJ/kg (for farming system (4)). The energy output-input ratio also increased from 2·8 to 5·9. It was also observed that farming system (3) consumed almost as much energy as system (4), yet the grain yield was low. This may be because of the high fertilizer dose, which was about 20% more than the recommended dose. It may, therefore, be concluded that the increased energy consumption in the form of agro-inputs increases the energy efficiency and grain yield, but that management and proper use of various energy sources and machinery also plays an equally important role.     

Estimation of zone effective-volume using tracer-gas techniques

The concentration-decay and pulse-injection tracer-gas techniques were used to evaluate the effective volume of a zone. Measurements of airflow rates were carried out in an environmental chamber using SF₆ as the tracer gas. Results showed that the flow rate estimated from the concentration-decay technique is about 4·5–14·3% higher than actual flow rate if the effective volume of the chamber is assumed to be equal to the physical volume. The effective volume, estimated from tracer-gas measurements, was found to be about 1·5–2·7% higher than the physical volume of the chamber.     

Effect of variable single cycle peak overload on fatigue life

Crack propagation experiments were conducted on 6061-T6 Al-alloy, for various overload ratios (1·75, 2·00, 2·15, 2·25 and 2·5). On the basis of these experiments a power law is developed to predict the delay period. The delay period after the application of a single overload increases as the magnitude of overload increases. Crack growth decreases after the application of an overload cycle and after a certain number of cycles it tends to attain its CAL crack propagation rate.     

Estimating the agricultural demand for electricity in the presence of measurement error in the data

This paper begins by discussing some of the problems frequently encountered in obtaining demand elasticity estimates. To these problems is added that associated with inaccuracy in the measurement of the dependent variable and one or more of the independent variables that impact upon the quantity demanded. Two diagnostics — the regression coefficient bounds and the bias correction factor — are introduced to assess the effect that such a measurement error has on the estimated coefficients of demand relationships. Use of these diagnostics aids in assessing the integrity of the estimates obtained. In considering the demand for electricity for irrigation and the demand for electricity for other (non-irrigation) uses by farmers in the USA, both the quantity demanded and the unit price data available for demand model estimation purposes contain measurement errors. The regression coefficient bounds diagnostic is used to indicate a range over which the true price responsiveness of farmers to changes in energy prices lies. The results suggest that each 1% increase (decrease) in the price of energy will result in between a 0·51 and 0·35% decrease (increase) in the quantity of electricity demanded for irrigation and between a 0·43 and 0·17% decrease (increase) in the quantity of electricity demanded for other uses. The bias correction factor is computed to evaluate the magnitude of the under-estimation of the responsiveness of the quantity of electricity demanded for irrigation and electricity for other uses to a change in the number of acres irrigated and the number of acres planted. For electricity for irrigation, the under-estimation was 14·1% while, for electricity for other uses, it was 13·9%.     

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.     

The effect of preheating of wood on ignition temperature of wood char

Samples of solid cottonwood in the form of discs have been heated at 250°C in both air and nitrogen, up to 12 h, and in air at 150°C up to 30 days. The preheated samples were then further heated in air at 5°C/min to unpiloted ignition. Despite major chemical changes during the various preheating treatments, the ignition temperatures were not significantly affected, except for a slight decrease (11 °C) after 5 days at 150°C in air.     

Investigation of yield strength and single cycle overload on crack closure

Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N_D/N_CAL vs overload ratio and crack length (when U = U_min) vs overload ratio fit a straight line, from which the power laws are developed.