R. Greif

R. Greif
University of California, Berkeley | UCB · Department of Mechanical Engineering

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236
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Publications

Publications (236)
Patent
This disclosure provides systems, methods, and apparatus related to an Organic Flash Cycle (OFC). In one aspect, a modified OFC system includes a pump, a heat exchanger, a flash evaporator, a high pressure turbine, a throttling valve, a mixer, a low pressure turbine, and a condenser. The heat exchanger is coupled to an outlet of the pump. The flash...
Article
In this work, the secondary flow and recirculation flow motions initiated at the bifurcations and bends in symmetric fractal-like microchannel networks with two branching levels were studied through three-dimensional simulations. The effects of Reynolds number and aspect ratio on the vortices generation were investigated. And the influences of thes...
Article
Based on our previous study, experiments were conducted to explore frictional pressure drop and heat transfer performance of de-ionized water flowing through rectangular microchannels having longitudinal vortex generators (LVGs). The experimental investigation was conducted under three-sided constant wall temperature boundary condition for the Reyn...
Article
Microreactors have important advantages due to the increased heat and mass transfer resulting from the small dimensions of the system. This results in enhanced performance and efficiency. Microreformers with wall-coated or suspended catalytic layer configurations have been studied due to their lower transport resistances compared to packed-bed micr...
Article
In this paper, the conventional equivalent method and the subsectional integral method were used to analyze the friction factor and the Nusselt number in fractal-like microchannel networks. The calculated results obtained from the two methods were compared with those obtained from the experimental data. It was proved that for complex configuration...
Article
In the present paper, the numerical simulations and experiments were carried out to explore the laminar hydrodynamic and thermal characteristics in the symmetrical fractal-like microchannel networks, and the pressure drop and heat transfer for the networks were compared with those for the S-shape and straight microchannels. To evaluate the comprehe...
Article
This paper focused on the hydraulic and thermal characteristics of fractal tree-like microchannels with different ARs (aspect ratios) for the Reynolds numbers ranging from 150 to 1200. And the fractal microchannels with two branches and straight microchannels with the same heat transfer area were also compared in terms of the pressure loss, mean he...
Article
This paper experimentally investigated the behavior of water through curved rectangular microchannels with different aspect ratios and curvature radii for Re numbers ranging from 10 to 600. The experimental data were compared with the results obtained from numerical analyses and the available correlations. The experimental results indicated that cl...
Article
Non condensing gas flow and heat transfer during venting of vessels are studied using experiments and analysis. A high pressure helium supply vessel is connected to a low pressure receiver via orifice and tubing. A single control volume analysis and a multi-dimensional analysis are used to predict pressure and mass-averaged temperature in the suppl...
Article
Full-text available
An analytical Detailed Loop Model (DLM) has been developed to analyze the performance of solar thermosiphon water heaters with heat exchangers in storage tanks. The model has been used to study performance of thermosiphons as a function of heat exchanger characteristics, heat transfer fluids, flow resistances, tank stratification, and tank elevatio...
Article
The solar concentration limit for densely packed, high-concentrated photovoltaic (HCPV) cells was analyzed for a novel two-phase cooling design. Eight working fluids were examined in the two-phase cooling analysis: R134a, R11, R113, R114, R123, R141b, water, and ammonia. In addition, the study investigated the concentration limit for mass flow rate...
Article
The ignition limits of hydrogen/air mixtures in turbulent jets are necessary to establish safety distances based on ignitable hydrogen location for safety codes and standards development. Studies in turbulent natural gas jets have shown that the mean fuel concentration is insufficient to determine the flammable boundaries of the jet. Instead, integ...
Article
The potential of increasing high-concentrator photovoltaic cell efficiency by cooling with two-phase flow is analyzed. The governing energy equations were used to predict cell temperature distributions and cell efficiencies for a photovoltaic cell under 100 suns' concentration. Several design conditions were taken into consideration in the analysis...
Article
In this paper, the flow characteristics in curved rectangular microchannels with different aspect ratios and curvature ratios for Re numbers ranging from 80 to 876 are investigated. The obtained experimental results are compared with simulated values based on classical Navier–Stokes equations and available correlation in the literature. An empirica...
Article
Full-text available
INTRODUCTION In many biological materials the energy transfer, in general, and the radiative transfer, in particular, are not well understood. The good insulation characteristics of Arctic mammals has led researchers to also study the effects of radiation heat transfer 1 . Previous research has focused on the bulk radiative properties of arctic ani...
Article
a b s t r a c t In the present study a small steam–methanol reformer with a colloid nanocatalyst is utilized to produce hydrogen. Radiation from a focused continuous green light laser (514 nm wavelength) is used to provide the energy for steam–methanol reforming. Nano-catalyst particles, fabricated by using pulsed laser ablation technology, result...
Article
A non-equilibrium wave based optical-thermal analysis was developed to study target heating and mass removal for irradiation with a femtosecond laser pulse utilizing a near-field scanning optical microscope probe. It was found that under the conditions when nano-craters were observed experimentally with a pure silicon target (Wen et al 2007 Appl. P...
Article
An experimental and analytical study of nanocatalyst fabrication and the production of hydrogen utilizing photon energy was carried out. The porous structure of the nanocatalyst provides a high surface to volume ratio of the catalytic layer thereby increasing the activity of the catalyst. An experiment was carried out to obtain an effective nanocat...
Article
a b s t r a c t An experimental and theoretical study of steam reforming of methanol is carried out in a packed-bed microreactor with internal heating. Experimental results of the methanol conversion and carbon monoxide concentration in an internally heated reformer are compared with those of an externally heated reformer. Higher methanol conversio...
Article
This study investigated the behavior of pressure drops and heat transfer coefficients for four sets of triangular microchannels. Results of this study indicated that for Reynolds numbers less than 100, the Nusselt number tends to be nearly linearly increasing as the Reynolds number increases. The product of the friction factor and the Reynolds numb...
Article
Significant laser energy was transferred from the near field scanning optical microscope tip to a silicon wafer producing nanopatterns on the sample surface. The patterns changed from nanoprotrusions to nanocraters when the background gas was changed from air to argon. Two physical mechanisms were attributed to this dramatic change, namely, oxidati...
Article
Steam reforming of methanol is investigated numerically considering both heat and mass transfer of the species in a packed bed microreactor. The numerical results are shown to be in good agreement with experimental data [M. Sources Transport in, 166 (2007) 194–201] with a BASF F3-01(CuO/ZnO/Al 2 O 3) catalyst. A correlation for the conversion effic...
Article
By fitting simulation results with experimentally measured trajectories of the shock wave and the vapor∕background gas contact surface, we found that inclusion of ionization energy in the analysis leads to a change in the evolution of the pressure, mass density, electron number density, and temperature of the vapor plume. The contribution of ioniza...
Article
Full-text available
Laser ablation has proven to be an effective method for generating nanoparticles; particles are produced in the laser induced vapor plume during the cooling stage. To understand the in situ condensation process, a series of time resolved light scattering images was recorded and analyzed. Significant changes in the condensation rate and the shape of...
Article
Heat transfer to and from a circular cylinder in a cross-flow of water at low Reynolds number was studied both experimentally and numerically. The experiments were carried out in a high aspect ratio water channel. The test section inflow temperature and velocity, channel lower surface temperature and cylinder surface temperature were controlled to...
Article
Full-text available
Time resolved shadowgraph images were recorded of shockwaves and particle ejection from silicon during laser ablation. Particle ejection and expansion were correlated to an internal shockwave resonating between the shockwave front and the target surface. The number of particles ablated increased with laser energy and was related to the crater volum...
Article
Full-text available
To study the expansion and cooling process of the laser induced plasma generated by nanosecond pulsed laser ablation, experiments have been conducted which measure the position of the external shockwaves and the temperature of the vapor plumes. The positions of external shockwaves were determined by a femtosecond laser time-resolved imaging system....
Article
Methanol-steam reforming can be utilized as a fuel processing system for hydrogen fuel cells. A study of the reacting flow in packed-bed and wall-coated catalytic reactors is presented. The wall-coated reformer has a smaller power requirement for delivering fuel than the packed catalytic bed reformer. Also, the coated catalytic layer has a smaller...
Article
A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and ene...
Article
Laser ablation of copper with a 4 ns laser pulse at 1064 nm was studied with a series of synchronized shadowgraph (100 fs laser pulses at 400 nm) and emission images (spectral line at 515 nm). Data were obtained at two laser pulse energies (10 and 30 mJ) and in three background gases (He, Ne, and Ar) at atmospheric pressure. The laser energy conver...
Article
A study was made of the cooling of the laser induced vapor plume in background air. The temperature and size variations of the vapor plume were determined from spectroscopic measurements during the first few tens of microseconds after the laser pulse. Experiments were carried out over a range of laser spot sizes and energies. The energy transport b...
Article
Full-text available
A study of the reforming rates, heat transfer and flow through a methanol reforming catalytic microreactor fabricated on a silicon wafer are presented. Packed bed microchannel reactors were fabricated using silicon DRIE, followed by wafer bonding. The reactor bed was subsequently filled with catalyst particles. Thermal control is achieved through o...
Article
The propagation of laser-induced shockwaves from ablation inside of cavities was determined from time-resolved shadowgraph images. The temperature and electron number density of the laser-induced plasma was determined from spectroscopic measurements. These properties were compared to those for laser ablation on the flat surface under the same energ...
Article
The proportion of zinc and copper in particles formed by laser ablation of brass was found to vary with the particle diameter. Energy-dispersive X-ray analysis showed that smaller particles were zinc enhanced while larger particles were composed mostly of copper. A model based on condensation of vapor onto large droplets ejected from a melted liqui...
Article
Femtosecond laser (Ti:sapphire, 100fs pulse duration) ablation of silicon in air was compared with nanosecond laser (Nd:YAG, 3ns pulse duration) ablation at ultraviolet wavelength (266nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was abo...
Article
Turbulent mixed convection heat transfer at high temperatures and large length scales is an important and seldom studied phenomenon that can represent a significant part of the overall heat transfer in applications ranging from solar central receivers to objects in fires. This work is part of a study to validate turbulence models for predicting hea...
Article
An experimental investigation is made into the fluid mechanics and heat transfer of a circular cylinder immersed in a wall-bounded turbulent mixed-convection flow of water. The cylinder is oriented spanwise to the forced channel flow and within the thermal boundary layer of the heated lower wall. The flow channel is capped with a cold, near-adiabat...
Article
Full-text available
Femtosecond laser ablation of silicon in air was studied and compared with nanosecond laser ablation at ultraviolet wavelength (266 nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater. The temperat...
Article
Propagation of the shock wave generated during pulsed laser ablation in cavities was measured using laser shadowgraph imaging and compared with laser ablation on a flat surface. The cavities were fabricated in fused silica with the same diameter and variable depths to provide aspect ratios (depth/diameter) of 3 and 6. It was found that outside the...
Article
Full-text available
The effect of laser energy on formation of a plasma inside a cavity was investigated. The temperature and electron number density of laser-induced plasmas in a fused silica cavity were determined using spectroscopic methods, and compared with laser ablation on a flat surface. Plasma temperature and electron number density during laser ablation in a...
Article
Femtosecond and nanosecond lasers were compared for ablating brass alloys. All operating parameters from both lasers were equal except for the pulse duration. The ablated aerosol vapor was collected on silicon substrates for particle size measurements or sent into an inductively coupled plasma mass spectrometer. The diameters and size distribution...
Article
This study investigates experimentally and numerically the characteristic of the pressure drop and the behavior of the fluids under the influence of the configurations of flow channel. The triangular microchannels have the transverse dimensions from 50µm to 100 µm; the fluid flowing through these channels has Reynolds numbers from 0.1 to 35. For th...
Conference Paper
Full-text available
An experimental and analytical study of the reacting flow in a catalytic reactor is presented. Methanol-steam reforming may be utilized in the fuel processing system for hydrogen fuel cells. Understanding the flow and transport phenomena as well as the reaction mechanisms is essential for improving the efficiency of the reforming process as well as...
Article
Cavity formation is a frequent result in many laser ablation applications. Although most theoretical investigations have been devoted to laser ablation on a flat surface, the development of a laser plasma inside a cavity is of both fundamental as well as practical significance. In this study, the temperature and electron number density of laser-ind...
Article
Most MEMS (microelectromechanical system) devices are made of silicon which is transparent at infrared wavelengths. Utilizing this infrared transparency of silicon, infrared thermal velocimetry was developed to measure the velocity in MEMS based fluidic devices. The method uses an infrared laser to generate a short heating pulse in a flowing liquid...
Article
The formation of a laser-induced plasma in a cavity and the effects of a cavity on the ablation process were investigated. Cavities were fabricated in fused silica with equal depths and variable diameters to provide aspect ratios (depth/diameter) of 1, 3 and 6. The temperature and electron number density of the pulsed laser-induced plasma in the ca...
Article
Full-text available
Measurement of velocity is essential for verification of the performance of many microelectromechanical system (MEMS) based fluidic devices. MEMS devices are usually made of silicon which is transparent at infrared wavelengths. Infrared thermal velocimetry has been developed to measure the velocity in silicon microchannels utilizing the infrared tr...
Article
A study of the heat and mass transfer, flow, and thermodynamics of the reacting flow in a catalytic microreactor is presented. Methanol reforming is utilized in the fuel processing system driving a micro-scale proton exchange membrane fuel cell. Understanding the flow and thermal transport phenomena as well as the reaction mechanisms is essential f...
Conference Paper
This study examines the mal-distribution problem in microchannel manifolds. The tubes are of triangular shape with hydraulic diameter of 25 and 50 μm. Ranges of the Reynolds number are from 0.1 to 9. The test results indicate that the mal-distribution decreases with the rise of flow rate. For an inlet flow rate of 0.1 mL/min at the distributor inle...
Article
A methodology for determining deposition with thermophoretic transport is introduced. An application is made to stagnation flow. The classical similarity solution for stagnation flow requires the boundary condition of a uniform particle concentration at an infinite location away from the wall. The present methodology removes these restrictions; i.e...
Article
Full-text available
The polar bear’s ability to survive in the harsh arctic night fascinates scientific and lay audiences alike, giving rise to anecdotal and semi-factual stories on the radiative properties of the bear’s fur which permeate the popular literature, television programs, and textbooks [1–5]. One of the most interesting radiative properties of polar bear f...
Conference Paper
Infrared thermal velocimetry is a new technique for the measurement of flow velocity. This non-intrusive technique was used to measure the velocity in a channel. In detail, a pulse of several milliseconds of a 5W CO2 laser generates a transient temperature response in a flowing liquid. Recording the radiative thermal images of the moving heated liq...
Conference Paper
As cooling requirements for electronic devices, e.g. computer processor units, power modules, etc. increase beyond the capabilities of air-cooling, interest has moved to several alternatives such as thermoelectric coolers, impinging jets and heat exchangers with phase change. Included among these, the capillary pumped loop is a very competitive coo...
Article
The ¯ow of liquid and vapor is investigated in trapezoidal and sinusoidal grooves. The e€ect of variable shear stress along the interface of the liquid and vapor is studied for both co-current and counter-current ¯ows. Velocity contours and results for the friction are obtained for both trapezoidal and sinusoidal grooves. An approximate relation th...
Article
Using picosecond time-resolved shadowgrams, we measured the development of an expanding air plasma during double picosecond pulse laser ablation of solids. Prior to the formation of the air plasma, a shock wave was established by the first laser pulse interaction with the solid target. The second picosecond laser pulse interacted with the shock wav...
Article
In the SBWR passive boiling water reactor, the long-term post-accident containment pressure is determined by the combination of noncondensible gas pressure and steam pressure in the wetwell gas space. The suppression pool (SP) surface temperature, which determines the vapor partial pressure, is very important to overall containment performance. The...
Article
Energy transport in diffusing systems is studied with special attention devoted to the transport that occurs as a result of mass diffusion,. Dimensionless parameters are presented for determining the importance of this transport mechanism; as an example of the application to heat and mass transfer, mixing of a binary gas in a heated, elliptic chann...
Article
As cooling requirements for electronic devices, e.g. computer processor units, power modules, etc. increase beyond the capabilities of air-cooling, interest has moved to several alternatives such as thermoelectric coolers, impinging jets and heat exchangers with phase change. Included among these, the capillary pumped loop is a very competitive coo...
Article
Infrared thermal velocimetry is a new technique for the measurement of flow velocity. This non-intrusive technique was used to measure the velocity in a channel. In detail, a pulse of several milliseconds of a 5W CO2 laser generates a transient temperature response in a flowing liquid. Recording the radiative thermal images of the moving heated liq...
Article
Picosecond time-resolved images of plasma initiation were recorded during pulsed-laser ablation of metal targets in an air atmosphere. An early-stage plasma was observed to form before the release of a material vapor plume. Close to the target surface, interferometry measurements indicate that the early-stage plasma has an electron number density o...
Article
Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than...
Article
A theoretical model has been developed to simulate plasma formation and evolution during the early stage of picosecond laser ablation of solids. Surface electron emission was implemented as one boundary condition for plasma development above the target. The simulation results indicate that a plasma forms, with electron density on the order of 1020...
Article
We present experimental observations of plasma formation during single picosecond laser pulse (35 ps, 1064 nm) ablation of copper in air. We studied an early-stage air plasma which forms before the emergence of a material vapor plume. Using picosecond time-resolved shadowgraphs and interferograms, we measured the evolution of this air plasma that h...