Abdullah M. Al-Turki

Comparative study on reduction of cooling loads by roof gravel cover

Abstract The increase in demand for power to meet the ascending energy used for air conditioning in arid areas calls for new techniques to lower the cooling loads. In this paper a simple inexpensive method is suggested, where the thermal resistance of the building roof is increased by adding a layer of loose gravel. The gravel properties as well as the intergranular trapped air reduces the heat transfer through the roof. This technique is investigated experimentally for actual outdoor weather conditions. The results show that the effect of the gravel mass intensity is more pronounced than the size of the gravel. A factor that relates the heat transfer through the gravel covered layer to that of blackened concrete roofs is suggested on the basis of the present experimental results.

Improving the yield of simple basin solar stills as assisted by passively cooled condensers

The performance of a simple solar still assisted by a passively cooled condenser is modeled and investigated both analytically and experimentally. The present analysis, based on actual meteorological data, shows that the yield improvement depends upon the heat removal rate by the condenser and brine depth that determines the thermal inertia of the still. For a still with negligible thermal storing capacity (brine depth approaches zero) an auxilliary condensing surface reduces the daily yield. The yield improves for stills with brine depth up to 5 cm, and increasing the condensing area helps to remove the vapor from the still and hence improves the productivity.

CALCULATION OF SHADING FACTOR FOR A COLLECTOR FIELD

A method is presented to calculate the instantaneous shading factor of any element in a collector field caused by the relative position of other collectors located in the successive southern row. The collector field consists of rows of collectors parallel to the east-west direction and oriented facing the equator. The method takes into account the variation in: latitude angle, day of the year, time of day, collector tilt angle, length to width ratio of the collector, spacing distance between rows, and number of collectors in each row. By means of geometrical analysis of a dimensionless field and by separation of variables, the calculation of the shading factor is carried out using a scale coefficient and two sets of basic shading factors. For the special case when the collectors are tilted with an angle equal to the latitude angle, and for a field using collectors with ratio of length to width equal to 2, a chart is prepared to predict the average row shading factor. An illustrative example is finally given to demonstrate the application of the method.

A study of reducing heat loads on tents due to solar insolation

Abstract The problem of heat transfer through the textile fabrics of tents due to solar radiation and ambient temperature variations is dealt with. This study is motivated by the extensive use of tents during Muslim pilgrimages when 2 million people live in tents (250000 tents) near Makkah (latitude 21° 29′) in Saudi Arabia, during extremely hot weather. The heat transfer rate due to combined solar insolation and ambient temperature variations is considered for a typical tent. The analysis is performed for periodic established and quasi-steady-state conditions. In search of passive means to reduce the heat rate, the direct beam solar radiation is eliminated by a canvas canopy. Experiments have been performed with a canopied east-facing attic-type roof, which showed that this simple arrangement reduces the daily heat rate by 46–49% for days with clear skies.

Comparison of the solar-operated two-stage and single-stage LiBrH2O absorption cycles

Abstract Analysis of a solar operated two-stage LiBrH 2 O cycle is presented. The cycle has the advantages that it can operate at a higher ambient temperature and at a lower generator temperature than those required by the single stage cycle. The performance of the cycle is predicted and compared to the performance of the single stage cycle at various design conditions. Four design parameters are identified to control the performance of the cycle. These parameters are the heat sink temperature, the evaporator temperature, the generator temperature, and the intermediate pressure. Numerical correlations are developed to specify the minimum allowable sink temperature, the minimum and maximum allowable generator temperature and the coefficient of performance of both the single-stage and the two-stage cycles. By defining a system thermal ratio for the combined absorption machine and a flat plate collector, the performances of the integrated systems are evaluated and compared when using both cycles. Conditions at which the performance of the solar operated two-stage cycle is superior to that of the solar operated single stage cycle are defined.

Modeling and simulation of compression ignition engines

ABSTRACT Modeling of a compression ignition engine was carried out covering losses emanent from imperfect construction of real engines such as progressive combustion, valve timing, and heat transfer. Furthermore, friction was included to obtain brake performance. Simulation of engine performance was tackled by varying engine speed, compression ratio and injection timing over wide range. The results were compared with those obtained from the experiemntal facility. Predictions by the model compare favourably with experiment within 9·3% and 9·7% for power and sfc respectively. The losses considered in this work amount to about 30% of the fuel energy input at the design point.

MODELING AND SIMULATION OF SPARK IGNITION ENGINES

ABSTRACT Modeling is important because it saves time, effort and cost needed for engine development and prediction of performance. In this work, losses due to imperfect construction of the real engine, including progressive combustion, valve timing and heat transfer have been modeled besides engine friction. Hence, it becomes possible to convert the output of the fuel-air cycle into net brake performance. Simulation of engine performance was carried out by varying engine speed, compression ratio and spark advance over wide range. Hence, it was possible to compare the results with those from experiments on a single cylinder engine. The model predictions were found to compare favourably with experiment within 4·6% in power and 2·9% in SFC. The losses considered in this work amount to about 14% of the fuel energy input.

Experimental investigation on free convection inside a tent envelope of textile fabric

Abstract Laminar free convection in an envelope enclosure of textile fabric is studied. The geometrical dimensions simulate those of small-capacity membraned-fabric buildings, in particular those of square-type tents. The convective heat transfer coefficients from a woven fabric (0.5 mm strand diameter, 10×10 strands/ cm 2 and 1 mm thick) roof and a similar metallic smooth surface were measured. The relation between Nusselt and Rayleigh numbers for 10 5 ⩽ Ra ⩽ 10 8 were developed for both surfaces at isoflux conditions. The present results showed that the heat transfer coefficient from the fabric surface of a tilted roof was two to three times higher than that for a similar smooth surface. Data was also obtained for the same tent but with two openings at the top and bottom (area ratio 1:4) to allow for ventilation air to pass through the tented cavity. For this ratio no significant effect upon the heat transfer coefficient from the roof was recorded.

Long term performance of a shaded collector field

A numerical model is presented to estimate the monthly average daily solar radiation absorbed by a collector field, with taking into account the effect of shading of the various collectors on each others. The field consists of NY rows of collectors, spaced uniformly by a distance Δy from each other, and each row contains. NX collectors. Each collector has a length L and width W and tilted towards the equator by an angle equals to the latitude angle of the location. The model accounts for collectors with one or two glass covers. Calculations are carried out using the model at various parametric values. The results are used to establish charts which can be used for a collector field of any dimensions and located at any location on the surface of the earth. A case study is presented to demonstrate the application of results of the present model to determine the optimum spacing between rows and the monthly average daily solar thermal energy absorbed by the collector field.