Hany Al-Ansary

Numerical and Experimental Analysis of Single-Phase and Two-Phase Flow in Ejectors

This paper presents results from an ongoing research effort to study single-phase and two-phase flow in ejectors numerically and experimentally. The goal of this study is twofold. The numerical analysis is intended to explore the details of the complex flow patterns within an ejector to help design more efficient ejectors. The experimental study was mainly intended to verify the feasibility of an idea previously proposed by the authors. This idea is to introduce fine droplets of a nonvolatile liquid into the driving fluid flow to reduce irreversibilities in the mixing chamber, thereby improving the overall efficiency of the ejector. The numerical results show that CFD analysis can be a very helpful and often reliable tool to understand the complex flow patterns within ejectors. Those results show some interesting flow phenomena that have not been adequately addressed in the literature. It was also found that CFD results are strongly dependent on grid resolution and the turbulence model employed. The experimental results show that two-phase flow can indeed be advantageous in certain cases. The limitations of this idea are discussed in some detail.

On-sun testing of an advanced falling particle receiver system

A 1 MWth high-temperature falling particle receiver was constructed and tested at the National Solar Thermal Test Facility at Sandia National Laboratories. The continuously recirculating system included a particle elevator, top and bottom hoppers, and a cavity receiver that comprised a staggered array of porous chevron-shaped mesh structures that slowed the particle flow through the concentrated solar flux. Initial tests were performed with a peak irradiance of ~300 kW/m2 and a particle mass flow rate of 3.3 kg/s. Peak particle temperatures reached over 700 °C near the center of the receiver, but the particle temperature increase near the sides was lower due to a non-uniform irradiance distribution. At a particle inlet temperature of ~440 °C, the particle temperature increase was 27 °C per meter of drop length, and the thermal efficiency was ~60% for an average irradiance of 110 kW/m2. At an average irradiance of 211 kW/m2, the particle temperature increase was 57.1 °C per meter of drop length, and the th...

The Effect of Alumina–Water Nanofluid on Natural Convection Heat Transfer Inside Vertical Circular Enclosures Heated from Above

Experimental investigation on natural convection heat transfer is carried out inside vertical circular enclosures filled with Al2O3–water nanofluid with different concentrations; 0.0%, 0.85% (0.21%), 1.98 (0.51%), and 2.95% (0.75%) by mass (volume). Two enclosures are used with 0.20 m inside diameter and with two different aspect ratios. The top surface of the enclosure is heated using a constant-heat-flux flexible foil heater while the bottom surface is subject to cooling using an ambient air stream. Various heat fluxes are used to generate heat transfer through the nanofluid. The average Nusselt number is obtained for each enclosure and correlated with the modified Rayleigh number using the concentration ratio as a parameter. A general correlation for the average Nusselt number with the modified Rayleigh number is obtained using the volume fraction and the aspect ratio as parameters to cover both enclosures. The results show that the Nusselt number for the alumina–water nanofluid is less than that of the base fluid. This means that using the alumina–water nanofluids adversely affects the heat transfer coefficient compared to using pure water. It is also found that the degree of deterioration depends on the concentration ratio as well as the aspect ratio of the enclosure.

Numerical Modeling of Natural Convection Heat Transfer around Horizontal Triangular Cylinders

Laminar natural convection heat transfer from uniformly heated horizontal cylinders of a triangular cross-section is investigated numerically in air. Different cylinder cross-section dimensions and orientations are examined under laminar conditions. The computational procedure is based on the finite volume technique (using the commercial software FLOWORKS). Results are presented in the form of streamline and temperature contour plots around the perimeter of the cylinders. Some representations of thermal layers around the side of the cylinders are obtained for different modified Rayleigh numbers. General correlations of Nusselt numbers verse the modified Rayleigh numbers are obtained. Symmetric, transition, and asymmetric plumes are obtained and characterized for both positions of the cylinders.

High Temperature Durability of Solid Particles for Use in Particle Heating Concentrator Solar Power Systems

Using solid particulates as a heat absorption and transfer medium in solar concentrated systems is a solution for collecting and storing thermal energy. Solid particulates, such as sand, are relatively inexpensive and are much less corrosive and expensive to maintain than molten salts. Small particles may be stored easily, and can be used as a heat transfer medium for use with a suitable heat exchanger. Despite their anticipated low cost, excessive degradation of the particulates requiring replenishment or disrupting operation could impair the overall economics. Consequently, the durability of the particulates should be verified. Responding to this need, this study examines the durability of solid particulates as a heat transfer medium in a closed cycle for concentrated solar power central receiver systems. Specifically, this study analyzes the combination of attrition and sintering of sand with varying temperatures. Attrition is the reduction of a particle’s mass and sintering is a process of fusing two or more particles together to form a larger agglomerate. In a closed cycle, particularly for a concentrated solar power tower, a particle will experience typical temperatures from 600°C to 1000°C. The increase in temperature may change the physical characteristics of the particles and along with any impurities may promote lower softening point bonding. Thus, it is important to investigate particle durability at high temperatures.The experimental procedure used in this investigation involves heating and abrading particulates of a known mass and size distribution to temperatures between 600°C and 1000°C, and also at 25°C to observe attrition only. The testing is conducted using a specially designed experimental apparatus described below. The heated particulates are contained in a metal cylinder. Inside the cylinder is another cylinder made of a porous silicon carbide foam. As the temperature is held constant, the particulate sample is rotated 180 degrees around a horizontal axis every 15 seconds from a low position to a higher position so that the particulates fall and abrade against each other. This process is repeated for a known number of cycles (many thousands). Then the resulting particulate size distribution is measured to determine the amount of attrition and sintering occurred during the experiment. The particulates tested are various types of sand with varying mean diameters and composition, along with a ceramic particulate similar to hydraulic fracturing proppants. Sample composition, sample size distribution, and temperature will be used to establish parameters for rates of attrition and sintering. These rates will be used to predict the behavior of particulates in a concentrated solar power tower closed cycle.Copyright

The Effect of Particle Concentration on Cooling of a Circular Horizontal Surface Using Nanofluid Jets

Here we report an experimental study of the heat transfer between a nanofluid jet and a horizontal heated circulate disk. The nanofluid jet has circular cross section and includes Al2O3 nanoparticles and water as the matrix fluid. The measurements include varying flow rates (0.006–0.075 kg/s) and jet diameters (3.9–8.2 mm) and nanoparticle concentrations (6.6 and 10% by mass). The Nusselt number increases for a given Reynolds number for increasing particle concentration, and this sensitivity is larger than that achieved through the variation in jet diameter and velocity explored in this study. Finally, the data are collapsed on one curve where the Peclet number is used for each concentration.

Desalination With a Solar-Assisted Heat Pump: An Economic Optimization

The solar-assisted heat pump (SAHP) desalination, based on the Rankin cycle, operates in low temperature and utilizes both solar and ambient energy. An experimental SAHP desalination system has been constructed at the National University of Singapore, Singapore. The system consisted of two main sections: an SAHP and a water distillation section. Experiments were carried out under the different meteorological condition of Singapore and results showed that the system had a performance ratio close to 1.3. The heat pump has a coefficient of performance of about 8, with solar collector efficiencies of 80% and 60% for evaporator and liquid collectors, respectively. Economic analysis showed that at a production rate of 900 L/day and an evaporator collector area of around 70 m2 will have a payback period of about 3.5 years.

Natural Convection Heat Transfer From Vertical Triangular Ducts

Experimental investigations have been reported on steady state natural convection from the outer surface of vertical triangular cross section ducts in air. Three ducts have been used with equilateral side length of 0.044, 0.06 and 0.08 m. The ducts are heated using internal constant heat flux heating elements. The temperatures along the vertical surface and the peripheral directions of the duct wall are measured. Axial (perimeter averaged) heat transfer coefficients along the side of each duct are obtained for laminar and transition to turbulent regimes of natural convection heat transfer. Axial (perimeter averaged) Nusselt numbers are evaluated and correlated using the modified Rayleigh numbers for laminar and transition regime using the vertical axial distance as a characteristic length. Critical values of the modified Rayleigh numbers are obtained for transition to turbulent. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers for all ducts. The local axial (perimeter averaged) heat transfer coefficients are observed to decrease in the laminar region and increase in the transition region. Laminar regimes are obtained at the lower half of the ducts and its chance to appear decreases as the heat flux increases.Copyright

Sun position algorithm for sun tracking applications

The efficiency of solar power systems, either concentrating or non-concentrating, can be increased by incorporating a tracking system. The tracking controller requires the position of sun in the sky which can be obtained by using a sun position algorithm that calculates sun position in a certain coordinate system. This papers presents a method to calculate the sun position which could be useful in tracking sun without sensors. The proposed algorithm is a compilation of different equations that compute solar position. The accuracy of the algorithm affects the efficiency of the tracking system but a complex high accuracy algorithm can be replaced with a simple low accuracy algorithm without affecting the accuracy to much extent as presented in this paper.

Heat Loss Experiments on a Non-Evacuated Parabolic Trough Receiver Employing a Thermally Insulating Layer in the Annular Gap

Parabolic trough collectors are economically and technically attractive options for process heat applications that require temperatures in excess of 200 °C. One of the reasons is that low-cost non-evacuated receivers are used in this type of application. However, at higher temperatures, the performance of non-evacuated receivers deteriorates considerably due to excessive radiation and natural convection losses. A new idea had been preliminarily investigated by the authors both numerically and experimentally. The idea was to introduce a thermally insulating layer to the part of the receiver’s annular gap that does not receive concentrated sunlight from the parabolic mirrors, and the results had been quite promising. This paper presents additional, more extensive experiments on this concept. In these experiments, a cartridge heater is inserted along the axis of the receiver tube of a non-evacuated receiver. The heater is surrounded by a conductive material to ensure uniform heating of the receiver tube. A number of thermocouples are affixed near the inner surface of the receiver as well as on the outer surface of the glass envelope to monitor temperature uniformity. Two sets of experiments are then conducted, one with the insulating layer, and the other without. In each set, the power input is set to a certain level and the receiver temperature is measured once steady state conditions are attained. The power level is then increased, and the measurements are repeated. The heat loss values from each set are compared to determine whether adding the insulating layer enhances receiver performance. Results show that a reduction in heat loss of as much as 15% can be achieved using this design, and collector efficiency can increase by up to about 6%. However, it was also found that the extent of improvement in collector efficiency depends on the operating temperature and direct normal irradiance, with the improvement being more significant at higher temperature applications and at low direct normal irradiance.Copyright