Khaled Gommed - Academia.edu (original) (raw)

Papers by Khaled Gommed

Journal of tribology, 1994

A parametric study is performed for a relatively large size helium cryocooler which utilizes a ri... more A parametric study is performed for a relatively large size helium cryocooler which utilizes a ringless piston. The results are presented in a dimensionless form and may be useful for other ringless-piston systems as well. Altogether twelve different parameters were studied and their effect on piston stability and sealing performance was analyzed and discussed. Although the cylindrical piston shape is commonly used for ringless piston applications, it seems that this is not the best choice for an optimum design.

Universidad Politécnica de Cartagena eBooks, 2015

The treatment of humidity on HVAC systems is crucial when a satisfactory indoor air quality needs... more The treatment of humidity on HVAC systems is crucial when a satisfactory indoor air quality needs to be reached. Traditional HVAC systems meet the latent cooling load by reducing the air temperature until its dew point, heating subsequently the air in order to reach the supply temperature for user comfort, with the energy waste this entails. On the other hand desiccant wheels requires normally an excessive post-cooling because of employed regeneration temperatures of around 70-80 ºC. In this paper the design and simulation models and testing results at laboratory scale of a hybrid liquid desiccant system (HLDS), developed in the frame of the EU project nanoCOOl, are presented. The HLDS is especially suited for applications with a low SHR (Sensible Heat ratio) and high ventilation requirements in tropical or subtropical climates. The aim of the project is to validate the developed technology for a good indoor environment quality, achieving the required ventilation needs, a good occupant comfort by the treatment of temperature and humidity to reach comfort conditions, avoiding the generation of moulds and microbial growth due to the antimicrobial properties of the LiCl. Detailed models of the HLDS components have been implemented in Engineering Equation Solver (EES) [1], and the whole model of the prototype has been developed, as well. The key parameters for the simulated HLDS, H&MTC (Heat&Mass transfer coefficients) have been experimentally obtained, testing the proof of concept absorber /regenerator in a test bench specially developed at laboratory scale[2]. The obtained values are in agreement with the correlations proposed by Bykov [3] (HTC) and Queiroz [4] (MTC).

Proceedings of International Conference CISBAT 2015 Future Buildings and Districts Sustainability from Nano to Urban Scale, 2015

arXiv (Cornell University), Dec 15, 2022

In the absence of gravity, surface tension dominates over the behavior of liquids. While this oft... more In the absence of gravity, surface tension dominates over the behavior of liquids. While this often poses a challenge in adapting Earth-based technologies to space, it can also provide an opportunity for novel technologies that utilize its advantages. In particular, surface tension drives a liquid body to a constantmean-curvature shape with extremely smooth surfaces, properties which are highly beneficial for optical components. We here present the design, implementation and analysis of parabolic flight experiments demonstrating the creation and in-situ measurement of optical lenses made entirely by shaping liquids in microgravity. We provide details of the two experimental systems designed to inject the precise amount of liquid within the short microgravity timeframe provided in a parabolic flight, while also measuring the resulting lens' characteristics in real-time using both resolution target-imaging and a Shack-Hartmann wavefront sensing. We successfully created more than 20 liquid lenses during the flights. We also present video recordings of the process, from the lenses' creation during microgravity and up until their collapse upon return to gravity. To the best of our knowledge, this is the first time that a purely liquid lens has been created in microgravity, which demonstrates the feasibility of creating and utilizing liquid-based optics in space.

Solar Energy, 2007

Growing demand for air conditioning in recent years has caused a significant increase in demand f... more Growing demand for air conditioning in recent years has caused a significant increase in demand for primary energy resources. Solarpowered cooling is one of the environmentally-friendly techniques which may help alleviate the problem. A promising solar cooling method is through the use of a liquid desiccant system, where humidity is absorbed directly from the process air by direct contact with the desiccant. The desiccant is then regenerated, again in direct contact with an external air stream, by solar heat at relatively low temperatures. The liquid desiccant system has many potential advantages over other solar air conditioning systems and can provide a promising alternative to absorption or to solid desiccant systems. Earlier work by the authors included theoretical simulations and preliminary experiments on the key components of the liquid desiccant system. The objective of the present study has been to construct a prototype system based on the knowledge gained, to monitor its performance, identify problems and carry out preliminary design optimization. A 16 kWt system was installed at the Energy Engineering Center at the Technion, in the Mediterranean city of Haifa. The system comprises a dehumidifier and a regenerator with their associated components operating together to dehumidify the fresh (ambient) air supply to a group of offices on the top floor of the building. LiClwater is employed as the working fluid. The system is coupled to a solar collector field and employs two methods of storage-hot water and desiccant solution in the regenerated state. The performance of the system was monitored for five summer months under varying operating conditions. The paper describes the operation of the experimental system and presents the measured data and the calculated performance parameters.

A computer code in a flexible and modular form developed for simulation of absorption systems mak... more A computer code in a flexible and modular form developed for simulation of absorption systems makes it possible to investigate various cycle configurations with different working fluids. The code is based on unit subroutines containing the governing equations for the system's components. The equations are linked by a main program according to the user's specifications to form the complete system. The equations are solved simultaneously, and fluid properties are taken from a property data base. The code is user oriented and requires a relatively simple input containing the given operating conditions and the working fluid at each state point. The user conveys to the computer an image of his or her cycle by specifying the different subunits and their interconnection. Based on this information, the program calculates: (1) the temperature, flow rate, concentration, pressure, and vapor fraction at each state point in the system; and (2) the heat duty at each unit, from which the coefficient of performance may be determined. The program has been used successfully to simulate a variety of single-stage, double-stage, and dual-loop heat pumps and heat transformers, with the working fluids LiBr-H2O,H2O-NH3, LiBr/H2O-NH3, LiBr/ZnBr2-CH3OH, and more.

Journal of Solar Energy Engineering-transactions of The Asme, May 1, 2004

International Journal of Refrigeration-revue Internationale Du Froid, 2003

ABSTRACT The importance of heat and mass transfer additives in absorption chillers and heat pumps... more ABSTRACT The importance of heat and mass transfer additives in absorption chillers and heat pumps has been recognized for over three decades. However, a universally accepted model for the mechanisms responsible for enhanced absorption rates has yet to be proposed. The Marangoni effect—an instability arising from gradients in surface tension at the liquid-vapor interface—is generally accepted as the cause of the convective flows that enhance transfer rates. Certain surfactant additives can significantly improve absorption rates and thus reduce the overall transfer area required by a given machine. Any means available that can increase the efficiency and acceptability of absorption machines is to be welcomed, as this technology provides an alternative to vapor compression systems which is both environmentally friendly and more versatile with regards to energy sources. This study investigates the rate at which a surfactant additive adsorbs at a liquid-vapor interface. The residence time of the falling liquid solution in an absorber is quite short. An effective additive must not only reduce the surface tension of the solution; it must do so quickly enough to cause the Marangoni instability within the short absorption process time. The entrance region of an absorber features a freshly exposed interface at which no surfactant has adsorbed. A numerical model is used to analyze surfactant relaxation rates in a static film of additive-laced solution. Kinetic parameters for the combination of the working pair LiBr-H2O and the additive 2-ethyl-1-hexanol are derived from data in the literature for static and dynamic surface tension measurements. Bulk, interfacial and boundary parameters influencing relaxation rates are discussed for surfactant adsorption occurring in the absence of absorption, as well as for concurrent adsorption and stable vapor absorption. Initial solution conditions and absorption driving force are shown to impact the potential for instability in the effect they have on the rate of interfacial additive adsorption.

Journal of tribology, Apr 1, 1993

A mathematical model is developed for analyzing the dynamics of gas lubricated ringless pistons. ... more A mathematical model is developed for analyzing the dynamics of gas lubricated ringless pistons. It takes into account the effects of piston-cylinder geometry, dynamic properties of piston and connecting rod, and operating conditions. A corresponding computer program is described which enables to obtain the piston trajectory and evaluate its stability. Preliminary results are presented for a reference case of a cryocooler which uses a stirling cycle design where helium gas is compressed by a motor driven piston. It is found that the connecting rod inertia plays an important role in stabilizing the piston motion. It is also found that the stability of piston motion strongly depends on the thermohydrodynamic condition of operation. Under isothermal condition the piston can be stable while under adiabatic condition its dynamic equilibrium may be unstable.

Flow

We present a method that leverages projected light patterns as a mechanism for freeform deformati... more We present a method that leverages projected light patterns as a mechanism for freeform deformation of a thin liquid film via the thermocapillary effect. We developed a closed-form solution for the inverse problem of the thin-film evolution equation, allowing us to obtain the projection pattern required in order to achieve a desired topography. We experimentally implement the method using a computer controlled light projector, which illuminates any desired pattern onto the bottom of a fluidic chamber patterned with heat–absorbing metal pads. The resulting heat map induces surface tension gradients in the liquid–air interface, giving rise to thermocapillary flow that deforms the liquid surface. If a polymer is used for the liquid film, it can then be photocured to yield a solid device. Based on the inverse-problem solutions and using this system, we demonstrate the fabrication of several diffractive optical elements, including phase masks for extended depth of field imaging, and for ...

Journal of Solar Energy Engineering, 2004

The growing demand for air conditioning, particularly in hot and humid climates has caused a sign... more The growing demand for air conditioning, particularly in hot and humid climates has caused a significant increase in demand for energy resources. A promising solar technology with potential to alleviate the problem is an open absorption system, where humidity is absorbed directly from the air to be treated by direct contact with the absorbent. The absorbent is then regenerated, again in direct contact with an external air stream, at relatively low temperatures of the heat source. The paper describes a study of a liquid desiccant cooling system designed to air-condition a group of offices on the top floor of a building in the Mediterranean city of Haifa, Israel. The system is capable of using as its source of power low-grade solar heat, of the type obtainable from low-cost flat plate collectors, and has a potential to provide both cooling and dehumidification in variable ratios, as required by the load. Several cycle variations have been considered, corresponding to different design ...

International Journal of Heat and Mass Transfer, 1997

This paper is concerned with the effect of non-absorbable gases on the combined heat and mass tra... more This paper is concerned with the effect of non-absorbable gases on the combined heat and mass transfer process in film absorption. Some simple models of the phenomenon have been proposed before which have under-predicted the mass flux. Here, a detailed theoretical model has been developed for the film, taking into consideration bulk movements in the vapor and liquid phases in

$Research paper thumbnail of Fabrication of diffractive optical elements by programmable thermocapillary shaping of thin liquid films$

Diffractive optical elements (DOEs) enable precise manipulation of wavefronts and are widely used... more Diffractive optical elements (DOEs) enable precise manipulation of wavefronts and are widely used in a variety of optical systems. However, their fabrication relies on lithography or high precision machining processes that are long, expensive, and infrastructure-heavy. We here present a one-step rapid fabrication method that leverages the thermocapillary effect to shape thin liquid films into useful DOEs with sub-nanometeric surface roughness. Our system consists of a projection system, which illuminates any desired pattern onto the bottom of a fluidic chamber patterned with heat-absorbing pads. The heat induces surface tension gradients in the polymer-air interface, resulting in the polymer film deformation. The polymer is then photocured to yield a solid device. We developed a theoretical model that provides the required projection pattern to achieve a desired topography. Based on this model, we demonstrate the fabrication of several DOEs, including phase masks for extended depth ...

Journal of Heat Transfer

Absorption heat transformers (AHTs) are a type of absorption heat pumps that are primarily driven... more Absorption heat transformers (AHTs) are a type of absorption heat pumps that are primarily driven by low-grade (typically waste) heat and produce higher temperature (high-grade) heat. Under the Indus3Es project, a 10 kW LiBr-H2O “Lab Scale” absorption heat transformer was built as a first experimental step toward larger scales. The focus was on the high-pressure vessel (HPV) (absorber and evaporator) design. To enhance performance, the aim was to obtain complete adiabatic absorption prior to the main absorption process accompanied by heat transfer. This maximizes the temperature within the absorber. This is particularly beneficial for absorption heat transformers, compared to chillers, because obtaining an elevated temperature is the objective. To obtain adiabatic absorption, atomizing spray nozzles were used as the liquid absorbent distribution system. This method proved successful; complete adiabatic absorption was obtained before the droplets contacted the absorber heat exchange ...

International Journal of Refrigeration

Physical Review Fluids

The study of thermocapillary driven flows is typically restricted to "open" systems, i.e., ones w... more The study of thermocapillary driven flows is typically restricted to "open" systems, i.e., ones where a liquid film is bounded on one side solely by another fluid. However, a large number of natural and engineered fluidic systems are composed of solid boundaries with only small open regions exposed to the surrounding. In this work we study the flow generated by the thermocapillary effect in a liquid film overlaid by a discontinuous solid surface. If the openings in the solid are subjected to a temperature gradient, the resulting thermocapillary flow will lead to a nonuniform pressure distribution in the film, driving flow in the rest of the system. For an infinite solid surface containing circular openings, we show that the resulting pressure distribution yields dipole flows which can be superposed to create complex flow patterns, and demonstrate how a confined dipole can act as a thermocapillary motor for driving fluids in closed microfluidic circuits. For a mobile, finite-size surface, we show that an inner temperature gradient, which can be activated by simple illumination, results in the propulsion of the surface, creating a thermocapillary surface swimmer.

Journal of Fluid Mechanics

The interaction of a thin viscous film with an elastic sheet results in coupling of pressure and ... more The interaction of a thin viscous film with an elastic sheet results in coupling of pressure and deformation, which can be utilized as an actuation mechanism for surface deformations in a wide range of applications, including microfluidics, optics and soft robotics. Implementation of such configurations inherently takes place over finite domains and often requires some pre-stretching of the sheet. Under the assumptions of strong pre-stretching and small deformations of the lubricated elastic sheet, we use the linearized Reynolds and Föppl–von Kármán equations to derive closed-form analytical solutions describing the deformation in a finite domain due to external forces, accounting for both bending and tension effects. We provide a closed-form solution for the case of a square-shaped actuation region and present the effect of pre-stretching on the dynamics of the deformation. We further present the dependence of the deformation magnitude and time scale on the spatial wavenumber, as w...