John Chai

Thermal mixing of two miscible fluids in a T-shaped microchannel.

In this paper, thermal mixing characteristics of two miscible fluids in a T-shaped microchannel are investigated theoretically, experimentally, and numerically. Thermal mixing processes in a T-shaped microchannel are divided into two zones, consisting of a T-junction and a mixing channel. An analytical two-dimensional model was first built to describe the heat transfer processes in the mixing channel. In the experiments, de-ionized water was employed as the working fluid. Laser induced fluorescence method was used to measure the fluid temperature field in the microchannel. Different combinations of flow rate ratios were studied to investigate the thermal mixing characteristics in the microchannel. At the T-junction, thermal diffusion is found to be dominant in this area due to the striation in the temperature contours. In the mixing channel, heat transfer processes are found to be controlled by thermal diffusion and convection. Measured temperature profiles at the T-junction and mixing channel are compared with analytical model and numerical simulation, respectively.

Examining Asphaltene Solubility on Deposition in Model Porous Media

Asphaltenes are known to cause severe flow assurance problems in the near-wellbore region of oil reservoirs. Understanding the mechanism of asphaltene deposition in porous media is of great significance for the development of accurate numerical simulators and effective chemical remediation treatments. Here, we present a study of the dynamics of asphaltene deposition in porous media using microfluidic devices. A model oil containing 5 wt % dissolved asphaltenes was mixed with n-heptane, a known asphaltene precipitant, and flowed through a representative porous media microfluidic chip. Asphaltene deposition was recorded and analyzed as a function of solubility, which was directly correlated to particle size and Péclet number. In particular, pore-scale visualization and velocity profiles, as well as three stages of deposition, were identified and examined to determine the important convection-diffusion effects on deposition.

A Total Concentration Method for Modeling of Deposition

This article presents a fixed-mesh total concentration method for modeling of deposition processes. It is adapted from the total concentration method initially proposed for etching process. The total concentration is defined as the sum of the concentration of the to-be-deposited (particles) and deposited materials (deposit). With this definition, the conservation equation for the total concentration, containing the interfacial conditions, can be derived and solved on a fixed mesh. The moving depositing front is captured implicitly by the concentration of the deposited material. The proposed method is validated against known exact solutions for a few one-dimensional deposition problems and solution from the level-set method for a two-dimensional problem. The method is used to investigate two-dimensional deposition without and with fluid flow coupled.

Parametric study of pool boiling from porous graphite foams in dielectric liquids

An experimental investigation of pool boiling on porous graphite foams is presented in this paper. A compact thermosyphon was developed to conduct the experiments using different types of graphite foams as evaporator inserts. Two dielectric liquids viz. FC-12 and HFE-700 were used as phase change coolants to investigate the effects of pore diameter and thermal conductivity of the foams on pool boiling heat transfer. It is found that the boiling heat transfer is affected by phase change coolant types. The difference of thermophysical properties between FC-12 and HFE-7000 resulted in different boiling characteristics on porous graphite foams. The experimental results indicate that thermal conductivity and pore diameter exhibit an interrelated effect on boiling performance. The compromise of these foam properties affects the bubble departure frequency which dominates the boiling heat transfer coefficient. This study demonstrates that the designed air-cooled thermosyphon can maintain the heater surface temperature below 85°C at heat flux of 112 W/cm2

Convective heat transfer in graphite foams with complex structures

Along with recent advances in electronic packaging, high performance computer processors require more efficient devices for dissipating the high heat fluxes. The use of highly conductive porous media has emerged as an effective cooling method due to its large internal contact surface area which enhances convection at the pore level. In this study, graphite foams developed at Oak Ridge National Laboratory, USA, are used to enhance heat transfer. Initial studies performed by other investigators showed that the extended surface area also results in very high pressure drop for fluid flowing through the graphite foam. This paper presents a numerical and experimental study of convection heat transfer in graphite foams with different structures which are designed to reduce pressure drop. The non local thermal equilibrium model is adopted in the fluid and solid energy equations. The numerical results which are validated by experimental data show that the inlet air flow partially penetrates the designed foam walls and the rest of the air flows tortuously through slots in the structure. Flow mixing is observed in the free stream area inside the structures, which is absent in block graphite foam. This indicates that better convection can be obtained by these structures due to their low flow resistance and high flow velocity. The pressure drop in the designed graphite foam is also found to be significantly lower than that in solid block graphite foam.

An evaluation of three spatial differencing schemes for the discrete ordinates method in participating media

Three popular spatial differencing schemes for the discrete ordinates method are examined for two-dimensional Cartesian coordinates system. These are a positive, the step, and the diamond schemes. Contrary to the common belief that negative intensities will not occur when fine spatial discretization is used with the diamond scheme, under certain conditions, the diamond scheme will produce negative intensities irrespective of the number of control volums employed. The positive scheme can produce physically unrealistic trends. The diamond and positive schemes are also capable of producing physically unrealistic overshoots. In absorbing-emitting or absorbing-emitting-scattering media, grid refinement can result in negative intensities when the diamond or positive scheme is used.

Experimental Study of Enhanced Pool Boiling Heat Transfer Using Graphite Foam Inserts

This paper presents the results of an experimental study of heat transfer in a pool boiling evaporator with porous insert. Different types of graphite foams were tested with the phase change coolant FC-72 in a designed thermosyphon. Comparisons between the graphite foams and a solid copper block show that the porous structure enhances pool boiling significantly. The boiling thermal resistance of the tested graphite foams was found to be about 2 times lower than that of the copper block. The bubble formation recorded by a high speed camera indicates that boiling from a graphite foam is more vigorous than from a copper block. The designed thermosyphon with graphite foam insert can remove heat fluxes of up to 112 W/cm2 with the maximum heater temperature maintained below 100°C.

Radiation heat transfer calculations using a control-angle, control-volume-based discrete ordinates method

A control-angle, control-volume-based discrete ordinates method (CA - CV DOM) is presented in this paper. A detailed formulation of the discretization equation is presented in two-dimensional Cartesian coordinate system. The procedure can be extended to curvilinear coordinate system with minor modifications. The step and modified-exponential schemes are used in this study. Present results converged to the grid independent solutions quickly and compared favorably against other published results for six test problems.

An Alternative Approach to Evaluate the Average Nusselt Number for Mixed Boundary Layer Conditions in Parallel Flow over an Isothermal Flat Plate

In this paper, we present an alternative approach to evaluate the average Nusselt number for mixed boundary layer conditions in parallel flow over an isothermal flat plate. This approach can be used regardless of the critical Reynolds number where the flow transitions from laminar flow to turbulent flow. This approach is simple and uses graphical visualisation of the physical situation. This should assist comprehension and retention. It utilises the average quantity for the laminar boundary layer and the average value for turbulent boundary layer to obtain the average quantities for mixed boundary layers without the need to perform the usual integration. It can easily be incorporated into part of undergraduate chemical, mechanical and petroleum engineering curricula. A worked example is included to show the utility of the approach.

Experimental study of asphaltene deposition in transparent microchannels using the light absorption method

ABSTRACT This study focuses on an experimental investigation of asphaltene deposition in a vertical transparent microchannel. Heptane-induced asphaltene precipitation is utilized to precipitate dissolved asphaltene in crude oil into asphaltene particles at ambient temperature and standard atmospheric pressure. These asphaltene particles deposit gradually on the surface of microchannels. The key parameters that influence the mechanism of asphaltene deposition are the ratio of crude oil to n-heptane and experimental elapsed time. At a constant flowrate, the amount of asphaltene deposited on a transparent channel wall is quantified using a new flow visualization technique based on reflected light intensity and image analysis. Asphaltene precipitation and deposition strongly affect the reflected light intensity through the change of mixture color in the recorded images. Experimental results show that asphaltene deposition process follows three stages, (i) slow asphaltene particle deposition at the beginning of the experiment, (ii) a rapid and continuous deposition occurring after few hours and (iii) a slower deposition (decreasing deposition rate) at the end of the experimentation. The experimental results for different crude oil to n-heptane ratios illustrate that deposition increases with this ratio, i.e. increasing concentration of n-heptane. An empirical equation is developed to correlate the intensity of the light absorption to the thickness of the deposited asphaltene in a transparent microchannel. Non-uniform deposition along the longitudinal direction of the microchannel is characterized. Deposits decrease with increasing longitudinal distance from the inlet. This non-uniform deposition distribution is due to local mass transport limitations and asphaltene aggregation size effect. GRAPHICAL ABSTRACT