H. Hegab

Friction and convection studies of R-134a in microchannels within the transition and turbulent flow regimes

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.

Fabrication and testing of a CoNiCu/Cu CPP-GMR nanowire-based microfluidic biosensor

Giant magneto resistance (GMR)-based microfluidic biosensors are used in applications involving the detection, analysis, enumeration and characterization of magnetic nano-particles attached to biological mediums such as antibodies and DNA. Here we introduce a novel multilayered CoNiCu/Cu nanowire GMR-based microfluidic biosensor. The current perpendicular to the plane of multilayers (CPP)-nanowires GMR was used as the core sensing material in the biosensor which responds to magnetic fields depending on the concentration and the flow velocity of bio-nano-magnetic fluids. The device was tested with different control solutions such as DI-water, mineral oil, phosphate buffered saline (PBS), ferrofluid, polystyrene superparamagnetic beads (PSB) and Dynabeads sheep anti-rabbit IgG. The nanowire array resistance decreased with an increase in the ferrofluid concentration, and a maximum 15.8% relative GMR was observed for the undiluted ferrofluid. The sensor was also responding differently to various ferrofluid flow rates. The GMR device showed variation in the output signal when the PSB and Dynabeads of different dilutions were pumped through it. When the tests were performed with pulsing potentials (150 mV and 200 mV), an increased GMR response was identified at higher voltages for PSB and Dynabeads sheep anti-rabbit IgG.

Living WITH the Lab - a freshman curriculum to boost hands-on learning, student confidence and innovation

A new freshman engineering curriculum has been implemented at Louisiana Tech University to boost hands-on learning, student confidence and innovation. The new curriculum, called living with the Lab, increases experiential learning by moving the ownership and maintenance of laboratory equipment from the university to the students. Each student purchases a robotics kit with a programmable controller, sensors, servos, and software along with a toolkit to provide the basis for a mobile laboratory and design platform. A basic tenet of the curriculum is that student-owned labs motivate student learning and broaden the spectrum of projects and design topics that can be addressed, thus facilitating innovation. The curriculum has been piloted for the past five years, and we are currently in the first year of full implementation to over 350 students thanks to a Phase II NSF CCLI grant. The paper presents the curriculum objectives, details of the three courses that make up the freshman curriculum, faculty training activities, and assessment results.

Performance of Counterflow Microchannel Heat Exchangers Subjected to External Heat Transfer

This article analyzes the effect of external heat transfer on the thermal performance of counterflow microchannel heat exchangers. Equations for predicting the axial temperature and the effectiveness of both fluids as well as the heat transferred between the fluids, while operating under external heating or cooling conditions, are provided in this article. External heating may decrease and increase the effectiveness of the hot and cold fluids, respectively. External cooling may improve and degrade the effectiveness of the hot and cold fluids, respectively. For unbalanced flows, the thermal performance of the microchannel heat exchanger subjected to external heat transfer depends on the fluid with the lowest heat capacity. At a particular number of transfer units (NTU), the effectiveness of both the fluids increased with decrease in heat capacity ratio when the hot fluid had the lowest heat capacity. When the cold fluid had the lowest heat capacity, the effectiveness of both fluids increased with decrease in heat capacity ratio at low values of NTU but at high values of NTU the effectiveness increased with increase in heat capacity ratio. A term called the “performance factor” has been introduced in this article to assess the relative change in effectiveness due to external heat transfer.

Experimental investigation of flow and heat transfer characteristics of R-134a in microchannels

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micro-milled in aluminum with hydraulic diameters ranging from approximately 112-mm to 210-mm and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Reynolds number, relative roughness, and channel aspect ratio were the parameters examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated transition from laminar to turbulent flow occurred between a Reynolds number of 2,000-4,000. Friction factor results were consistently lower than values predicted by macroscale correlations. Nusselt number results indicated channel size may suppress turbulent convective heat transfer. Results also indicate that surface roughness may affect heat transfer characteristics in the turbulent regime.

Characteristic Study on the Optimization of Pin-Fin Micro Heat Sink

The need for dissipating heat from microsystems has increased drastically in the last decade. Several methods of heat dissipation using air and liquids have been proposed by many studies, and pin-fin micro heat sinks are one among them. Researchers have developed several effective pin-fin structures for use in heat sinks, but not much effort has been taken towards the optimization of profile and dimensions of the pin-fin. In this paper the authors studied the effect of different pin-fin shapes on the thermal resistance and pressure drop in a specific micro heat-sink. Optimization subjected to two different constraints is studied in this paper. The first optimization is subjected to constant flow rate and the second one is subjected to constant pressure drop. Both optimization processes are carried out using computer simulations generated using COVENTORWARE™. Two of the best structures from each of these optimization studies are selected and further analysis is performed for optimizing their structure dimensions such as width, height and length. A section of the total micro heat-sink is modeled for the initial optimization of the pin-fin shape. The model consists of two sections, the substrate and the fluid. Six different shapes: square, circle, rectangle, triangle, oval and rhombus were analyzed in the initial optimization study. Preliminary tests were conducted using the first model described above for a flow rate of 0.6ml/min. The non dimensional overall thermal resistance of the heat sink, and the nondimensional pumping power was calculated from the results. A figure of merit (FOM) was developed using the nondimensional thermal resistance and nondimensional pumping power for each structure with different pin-fin shapes. Smaller the value of FOM better the performance of the heat sink. The study revealed that the circle and ellipse structures have the best performance and the rectangle structure had the worst performance at low flow rates. At high flow rates rectangular and square structures have the best performance.© 2009 ASME

Experimental Validation of a Thermal Model of Counterflow Microchannel Heat Exchangers Subjected to External Heat Flux

The effect of uniform external heat flux on the effectiveness of counterflow microchannel heat exchangers is experimentally studied in this article for validating an existing thermal model. The model validated in this study is a one-dimensional model previously developed by the same authors. The model is validated to be independent of microchannel profile, hydraulic diameter, and heat capacity ratio. For studying the effect of microchannel profile, experiments are conducted under balanced flow conditions using trapezoidal and triangular microchannels with approximately equal hydraulic diameter of 278.5 μm and 279.8 μm, respectively. The influence of hydraulic diameter on the thermal model is studied using a trapezoidal microchannel with hydraulic diameter of 231 μm and 278.5 μm. Experiments are conducted under unbalanced flow conditions, with a heat capacity ratio of 0.5, using the trapezoidal microchannel of hydraulic diameter of 278.5 μm. Deionized water is used as the fluid in all experiments. The hot and cold fluid effectiveness is studied and the theoretical predictions and experimental results are found to be in excellent agreement. Thus, the model validated in this article can be used for accurately modeling microchannel heat exchangers irrespective of the microchannel hydraulic diameter, profile, and heat capacity ratio.

Characteristic Study on the Optimization of Micro PinFin Heat Sink with Staggered Arrangement

The effect of the Pinfin shapes on the overall per formance of the micro Pinfin heat sink with staggered arrangement is studied in this paper. Six different shapes of micro Pinfins, square, rectangle, circle, rhombus, triangle and ellipse are subjected to study in this paper. The optimization processes are carried out using computer simulations performed using COVE*TORWARE™. The study is carried out over a range of Reynolds number ranging from 50 to 500 and a figure of merit term (FOM) consisting of both the thermal resistance and the pumping power is developed for the overall performance evaluation of different models. A weighted average scheme is used for developing the FOM term. The results of the study revealed that at low values of Reynolds numbers (Re 200) rectangle showed the best performance. *omenclature C = specific heat capacity (J/kg K) h = total height of the heat sink (m) k = thermal conductivity (W/K m) L = length of the heat sink (m) n = constant P = pressure (N/m 2 )

Microfabrication of nanowires-based GMR biosensor

This study focuses on the development of current-perpendicular-to plane (CPP) Giant Magnetoresistance (GMR) of CoNiCu/Cu multilayered nanowire based microfluidic sensors for the detection of magnetic nanoparticles and fluids. The visible measurable variations in electrical voltage due to changes in external magnetic field are later to be monitored in microfluidic biosensor for the detection of toxicants in cells. An early prototype device was fabricated and tested using both an aqueous nonmagnetic medium (water) and a commercially available ferrofluid solution. A magnetic field of 0.01T caused a resistance change of 1.37% for ferrofluid, while a 1.1% GMR was recorded for the water baseline.

Axial Heat Conduction in Parallel Flow Microchannel Heat Exchangers

This paper deals with the effect of axial heat conduction on the hot and cold fluid effectiveness of a balanced parallel flow microchannel heat exchanger. The ends of wall separating the fluids are subjected to Dirichlet boundary condition. This leads to heat transfer between the microscale heat exchanger and its surroundings and thereby leading to axial heat conduction through the wall separating the fluids. Three one dimensional energy equations were formulated, one for each of the fluids and one for the wall. These equations were solved using finite difference method. The effectiveness of the fluids depends on the NTU, axial heat conduction parameter, and the temperature of the ends of the wall separating the fluids. With decrease in temperature of the end wall at the inlet section of the fluids, while keeping the temperature of the other end wall constant, the effectiveness of the hot and cold fluid increased and decreased, respectively. When the temperature at the ends of the wall separating the heat exchanger is average of the inlet temperature of the fluids then there is no axial heat conduction through the heat exchanger. The effectiveness of a counter flow microchannel heat exchanger is better than that of a parallel flow microchannel heat exchanger subjected to similar operating conditions, i.e. axial heat conduction parameter and end wall temperatures.© 2009 ASME