Walid Chakroun

Pressure drop and heat transfer comparison for both microfin tube and twisted-tape inserts in laminar flow

Abstract Experiments were carried out to compare pressure drop and heat transfer coefficients for a plain, microfin, and twisted-tape insert-tubes. The twisted-tape experiments include three different twist ratios each with two different widths. The data were taken at Reynolds numbers well in the laminar region. The heat transfer data were obtained in a single shell-and-tube heat exchanger where steam is used as a heat source to obtain a uniform wall temperature and the working fluid in the tube is oil. The twist ratio and the width of the tape seem to have a large effect on the performance of the twisted-tape insert. The results demonstrate that as the twist ratio decreases, the twisted-tape will give better heat transfer enhancement. The loose-fit (W=10.8 mm) is recommended to be used in the design of heat exchanger where low twist ratios (Y=5.4, and Y=3.6) and high pressure drop situations are expected since it is easier to install and remove for cleaning purposes. Other than these situations, the tight-fit tape gives a better performance over the loose-fit tape. For the microfin tube tested in this paper, the data shows a small increase in both heat transfer and pressure drop. This type of microfin tube is not recommended to be used in laminar flow conditions.

Effect of tube-tape clearance on heat transfer for fully developed turbulent flow in a horizontal isothermal tube

Abstract This paper presents the effect of tube-tape clearance on the heat transfer for a fully developed turbulent flow in an isothermal tube. Tests were performed on 15 different tapes; they include 3 twisted tape ratios each with five different widths. Results have demonstrated that as the tube-tape clearance decreases, the heat transfer enhancement increases. For the case of twist ratio of 3.6, the tape widths have resulted in almost 17% difference in the enhancement. On the other hand, for the cases of twist ratios of 5.4 and 7.1, the enhancements were 9% and 4%, respectively. For practical design of thermal systems, operating under turbulent flow condition, small twist ratio and tight-fit tape are desirable in order to obtain a reasonably high heat transfer enhancement. The heat transfer for twist ratio of 3.6 and loose-fit tape shows nearly the same enhancement as the case of the tight-fit tape for the same twist ratio. Such behavior indicates the presence of an optimum tape width, and such width is a function of the twist ratio and Reynolds number

THERMAL COMFORT ANALYSIS INSIDE A CAR

Owing to the hot harsh weather conditions in Kuwait during the summer months, the ambient temperature inside a car parked in the sun may reach 50°C and the temperature may reach 75°C. As a result, people are seriously concerned about how to keep their cars cool. This paper addresses the behaviour of the air temperature inside a car parked in the sun. Different covering arrangements were tested, including covering the front windshield, covering the front windshield and four side windows, and covering both windshields and four side windows. In addition, the novel idea was also tested of installing a solar powered ventilating fan. Such a technique was found to be very effective in reducing the inside air temperature of a car parked in the sun. The effects of the covering arrangements of the car and the solar fan on the initial cooling rate of an air-conditioning system during normal driving conditions were also analysed. Different physical parameters, namely air velocity, mean radiant temperature and humidity, were combined with air temperature to determine the comfort level of a person inside a car.

Effect of Surface Roughness on the Aerodynamic Characteristics of a Symmetrical Airfoil

The objective of this study is to investigate the effect of surface roughness by varying the roughness size and location on the aerodynamic characteristics of the airfoil. Tests were conducted on the symmetrical airfoil models NACA 0012 on which the nature of the surface was varied from smooth to very rough and at a chord Reynolds number of 1.5*105. Different airfoil models with various roughness sizes and roughness locations were tested for different angles of attack. Lift, drag and pressure coefficients were measured and velocity profiles were determined for the smooth and grit 36 roughened models. It is shown that as the surface roughness increases, the minimum drag also increases due to the increase of the skin friction and the lift decreases. Surface roughness is seen to delay the stall angle and also increase the lift in the stall region. The airfoil model with the roughness located at the trailing edge shows minimum drag and maximum lift up to the stall angle compared to the other cases of different roughness locations. It is confirmed that, for the rough surface, a turbulent boundary layer exists where the laminar boundary layer is encountered for the smooth surface at the same Reynolds number. The measured skin friction for the rough surface is larger than that for the smooth surface.

The recent air temperature rise in Kuwait

Recently, there is increasing concern in Kuwait regarding the frequent occurrences of harsh climate conditions with summer air temperature exceeding the 50°C mark. In particular, in the last 20 years, the maximum yearly temperature is persistently exceeding its mean value for the whole recorded period.

Heat transfer measurements for smooth and rough tilted semi-cylindrical cavities

Abstract This paper presents the effect of roughness on heat transfer for semi-cylindrical cavity. Heat transfer measurements are performed on smooth and rough surfaces for different tilt angles. The results are compared with a rectangular cavity of the same surface area to study the effect of cavity shape on heat transfer. The roughness composed of 0.002 m diameter rods installed along the length of the cavity at equal spacing. Roughness shows a large effect on heat transfer for the semi-cylindrical cavities. Two competing effects are present with the existence of roughness. Roughness may increase the blockage effect on the flow that can cause the buoyancy force to decrease, but on the other hand it increases the turbulence intensity resulting in a higher heat transfer. Both effects are function of tilt angles. Heat transfer for the cylindrical cavity is higher than the rectangular cavity for all tilt angles. This increase in heat transfer for the cylindrical cavity is due to the absence of sharp corners that can slow the buoyancy driven convection mechanism.

Bias error reduction using ratios to baseline experiments - Heat transfer case study

Concluding Remarks In this study, numerical simulations by means of a finite difference method have been performed to render the effects of the relevant physical parameters of the problem considered (Re, Gr, and (/>) on the interaction between the shear-driven flow and the buoyant recirculating flow during the heating process of the moving plate. Results indicate that the sheardriven flowfields in the divided subchannels can be strongly affected by the buoyancy force due to an increase of Grashof number; a bicellular recirculation arises respectively within the subchannels in the region around the heaters. Moreover, an increase in Grashof number tends to promote the upstream diffusion effect due to the buoyant recirculation, particularly for the upper channel. Accordingly, the local Nusselt number on the top surface of the moving plate becomes less localized shifting in the upstream direction. Moreover, the flowfield and temperature distribution in the upper subchannel are found to be more sensitive to the inclination of the duct. For the range of inclination considered, the temperature as well as the heat transfer on the moving plate are rather unaffected by the variation of the duct orientation.

Heat transfer from rough, protruding discretely heated strips with and without flow acceleration

Abstract Experimental heat transfer results on the effects of rough, protruding, discretely heated strips are presented. Data are presented for two-dimensional (2-D), distributed heat sources for three cases—flush-mounted smooth, protruding smooth, and protruding rough—under zero pressure gradient and accelerated turbulent boundary-layer conditions. For the zero pressure-gradient cases, experiments are performed on repeated, protruding discretely heated strips using surface roughness as a heat transfer enhancement technique. The accelerated cases are conducted to investigate the existence and strength of a synergistic interaction between boundary-layer flow acceleration and surface roughness as a heat transfer enhancement mechanism. For the zero pressure-gradient cases, it is found that under the conditions of these experiments, significant heat transfer enhancements (10–100%) can be obtained with the rough surface. For the smooth-flush arrangement with acceleration, the Stanton numbers decreased by up to 16% with increasing acceleration strength. For the protruding smooth arrangement, the Stanton numbers remain almost constant over all acceleration strengths. For the protruding rough arrangement, the Stanton numbers increase by up to 7% with increasing acceleration strength.

Evaluation of Human Thermal Comfort in Offices in Kuwait and Assessment of the Applicability of the Standard PMV Model

It has been known that the human thermal comfort is not exclusively a function of air temperature but also a function of six additional parameters, namely, mean radiant temperature, air velocity, turbulence intensity, humidity, activity level, and clothing insulation. The combined physical and psychological impact of these parameters on thermal comfort is mathematically described in various comfort models. The current comfort models, while use extensive human comfort data, may not be applicable in all world regions due to environmental conditions and people’s expectations. The State of Kuwait has a population of 2.5 million inhabitants with majority of people living in a few populated cities with heavy vehicle traffic, office buildings, factories, petroleum operations, and shopping centers. During the summer months (especially in July and August) the temperature reaches 48 °C in the afternoon, and can sometimes exceed 55 °C requiring extensive use of air conditioning. The traditional clothing (Disdasha) is made of lightweight, white, fabric material to provide some level of comfort. To better understand the regional preferences and assess the applicability of the standard comfort models in Kuwait, important parameters influencing human thermal comfort were measured in ten different government offices and the corresponding PMV indices were calculated. The results were compared with other comfort indices to obtain the most viable comfort index and the appropriate temperature range for local comfort for Kuwait offices. This study is not only important for comfort evaluations but also for evaluation of energy consumption in office buildings.© 2004 ASME

Investigation of the effect of a smooth strip on rough-wall turbulent boundary layers

A series of experiments was conducted to study the effects of a smooth strip on heat transfer and fluid dynamics in the turbulent boundary layer on an otherwise rough surface. The first 0.9m of the test section is rough, followed by 0.1-m smooth strip, and the remaining 1.4m is rough. The rough surface is composed of 1.27-mm diameter hemi-spheres spaced 2-diameters apart in staggered arrays. The experiments include measurements of Stanton number distributions as well as mean temperature, mean velocity, and turbulence intensity profiles. The results are compared with previously published data from experiments with a rough leading portion and a smooth final portion and from experiments on an all-rough surface. Over the smooth strip, Stanton number decreases by almost 45% relative to the all-rough value; however, the Stanton number distribution recovers typical rough-wall behavior in a short distance. The Stanton number measurements are compared with predictions using the discrete-element method. In general, the agreement is excellent. Mean velocity and turbulence intensity profiles show the flow downstream of the strip to rapidly attain rough-wall behavior in the near region, while requiring more distance to exhibit a complete rough-wall behavior.