Sajjad Bigham

Role of bubble growth dynamics on microscale heat transfer events in microchannel flow boiling process

For nearly two decades, the microchannel flow boiling heat transfer process has been the subject of numerous studies. A plethora of experimental studies have been conducted to decipher the underlying physics of the process, and different hypotheses have been presented to describe its microscopic details. Despite these efforts, the underlying assumptions of the existing hypothesis have remained largely unexamined. Here, using data at the microscopic level provided by a unique measurement approach, we deconstruct the boiling heat transfer process into a set of basic mechanisms and explain their role in the overall surface heat transfer. We then show how this knowledge allows to relate the bubble growth and flow dynamics to the surface heat flux.

A Numerical Study on Natural Convention Heat Transfer From a Horizontal Isothermal Cylinder Located Underneath an Adiabatic Ceiling

A numerical analysis is performed for steady-state and two-dimensional natural convection heat transfer from a horizontal isothermal cylinder located underneath a horizontal adiabatic ceiling. The finite-volume method based on the SIMPLE algorithm and a nonorthogonal grid discretization scheme are used to solve the continuity, momentum, and energy equations for the Rayleigh numbers in the range from 10−1 to 104. The Poisson equations are solved to find the grid points, which are distributed in a nonuniform manner with higher concentration close to the solid regions. In addition, the HYBRID differencing scheme is used for the approximation of the convective terms in the curvilinear coordinate. The effects of the Rayleigh numbers and cylinder spacing from the adiabatic ceiling on both the local and average Nusselt numbers around the cylinder are investigated. Numerical results are performed for the plate-to-cylinder spacing ranging from 0.1 to 1.4.

Physics of microstructures enhancement of thin film evaporation heat transfer in microchannels flow boiling

Performance enhancement of the two-phase flow boiling heat transfer process in microchannels through implementation of surface micro- and nanostructures has gained substantial interest in recent years. However, the reported results range widely from a decline to improvements in performance depending on the test conditions and fluid properties, without a consensus on the physical mechanisms responsible for the observed behavior. This gap in knowledge stems from a lack of understanding of the physics of surface structures interactions with microscale heat and mass transfer events involved in the microchannel flow boiling process. Here, using a novel measurement technique, the heat and mass transfer process is analyzed within surface structures with unprecedented detail. The local heat flux and dryout time scale are measured as the liquid wicks through surface structures and evaporates. The physics governing heat transfer enhancement on textured surfaces is explained by a deterministic model that involves three key parameters: the drying time scale of the liquid film wicking into the surface structures (τd), the heating length scale of the liquid film (δH) and the area fraction of the evaporating liquid film (Ar). It is shown that the model accurately predicts the optimum spacing between surface structures (i.e. pillars fabricated on the microchannel wall) in boiling of two fluids FC-72 and water with fundamentally different wicking characteristics.

Microscale Layering of Liquid and Vapor Phases Within Microstructures for Self-Regulated Flow Delivery to Local Hot Spots

In this study, a new two-phase heat sink architecture is introduced that operates in two different phase change modes. At low wall superheat temperatures, the heat sink operates at the thin film evaporator mode and transitions to boiling when the wall superheat temperature is increased. This unique function is enabled through constraining the liquid and vapor phases into separate domains using capillary-controlled meniscus formed within a hierarchical 3D structure. The structure is designed to form thin layers of vertically oriented liquid films that directly evaporate into their neighboring vapor space. The dominant mode of heat transfer in this design is thin film evaporation, a very effective boiling sub-process. As the surface superheat temperature is increased and boiling starts, the capillary-controlled meniscus breaks down. A heat transfer coefficient of greater than 200 kW/m2K is achieved at less than 1 °C wall superheat temperature.Copyright

Physics of Interfacial Heat Transfer Events in Flow Boiling of FC-72 Liquid in Microchannels

This work examines the microscale physics of heat transfer processes in flow boiling of FC-72 in a single microchannel. Experimental results discussed in this paper provide new physical insight on the nature of heat transfer events. The study is enabled through development of a device with a composite substrate that consists of a high thermal conductivity material coated by a thin layer of a low thermal conductivity material with embedded temperature sensors. This novel arrangement enables measurement of local heat flux with a spatial resolution of 40–65 μm and a temporal resolution of 50 μs. The device generates isolated bubbles from a 300 nm in diameter artificial cavity fabricated at the center of a pulsed function micro-heater. Analysis of the temperature and heat flux data along with synchronized images of bubbles show that four mechanisms of heat transfer are active as a bubble grows and flows through the channel. These mechanisms of heat transfer are 1) microlayer evaporation, 2) interline evaporation, 3) transient conduction, and 4) micro-convection. The magnitude and time period of activation of these mechanisms of heat transfer are determined and their characteristics are discussed in details.Copyright

A Microscale Study of the Impact of Bubble Growth Dynamics on Surface Heat Flux in Microchannel Flow Boiling Process

In this study, the physics of microscale heat transfer events at the wall-fluid interface during the growth of a moving bubble in a microchannel is analyzed. The study is enabled through development of a novel device that utilizes 53 microscale platinum resistance temperature detectors (RTDs) embedded in a composite substrate made of a high thermal conductivity material coated by a thin layer of a low thermal conductivity material. This sensors arrangement enables resolving the thermal field at the bubble-wall interface with unprecedented spatial and temporal resolutions of 40–65 μm and 50 μs, respectively. To prevent random bubble inception, a 300 nm in diameter cavity is fabricated using a focused ion beam (FIB) at the center of a pulsed function microheater. A detailed analysis of the surface heat transfer events and their relations to time scale of formation and dimensions of bubbles are conducted to decipher the underlying physics of the flow boiling process. Experimental results show that four mechanisms of heat transfer are active as a bubble grows and flows through the channel. These mechanisms of heat transfer are 1) microlayer evaporation, 2) interline evaporation, 3) transient conduction, and 4) micro-convection. The results suggest that the average surface heat flux enhances as the bubble grows in size resulting in expansion of the surface area over which the thin film evaporation mechanism is active. Above a certain bubble size, the average surface heat flux declines due to the formation of a dry region at the bubble-wall interface. Hence, the results indicate that there is an optimal bubble length at which the average surface heat flux is maximum.Copyright

Absorption Characteristics of Multilayered Thin Lithium Bromide (LiBr) Solution Film

In this study, an alternative absorber design suitable for the plate-and-frame absorber configuration is introduced. The design utilizes a fin structure installed on a vertical flat plate to produce a uniform solution film and minimize its thickness and to continuously interrupt the boundary layer. Using numerical models supported by experiments employing dye visualization, the suitable fin spacing and size and wettability are determined. The solution flow thickness is measured using the laser confocal displacement measurement technique. The new surface structure is tested in an experimental absorption system. An absorption rate as high as 6×10−3 kg/m2s at a driving pressure potential of 700 Pa is achieved, which is considerably high in comparison with conventional absorption systems. The effect of water vapor pressure, solution flow rate, solution inlet concentration, cooling water inlet temperature and solution inlet temperature on the absorption rate is also investigated. The proposed design provides a potential framework for development of highly compact absorption refrigeration systems.Copyright