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Research Areas

Our research group conducts research in micro/nano-scale two-phase heat and flow physics, prediction, control and modeling. we aim at fundamental research in developing and verifying theories for two-phase transport behaviors and the application of these theories towards controlling two-phase heat and flows.



Flow Boiling

1- Bubble/Vapor Slug Dynamics in a Confined Domain

Bubble dynamics is very important to the fundamental understanding of flow boiling in microchannels. Bubble dynamics associates with bubble nucleation from heated surface, bubble growth rate, departure diameter and frequency, and condensation of bubble in subcooled liquid. The aim of study of bubble dynamics is to determine HTC at L-V interface through experimental and numerical study of bubble/vapor slug dynamics in a confined domain. Finally, a model will be developed for prediction of HTCs at liquid-vapor interface.

Bubble/vapor slug dynamics in a confined microchannel at a mass flux of 83.3 kg/m2 s and heat flux of 1560 W/cm2, replayed at 5 fps

2- Enhanced Flow Boiling in Microchannels by Regulating Two-phase Transport Patterns

Flow boiling in microchannels is one of the most promising cooling techniques for microelectronics. Using latent heat by vaporization can significantly improve heat dissipation of high power density electronic devices. However, the vigorous rapid generation of vapor through phase change leads to chaotic two-phase flows in microchannels, resulting in flow instability in terms of severe flow, temperature and pressure drop fluctuations. Particularly, the very well-known bubble confinement exacerbates the two-phase flow instabilities and greatly deteriorates heat transfer performance in terms of CHF and heat transfer coefficient (HTC). The regulation of two-phase transport is essential to improve flow boiling in microchannels. Two new concepts, which aim to radically solve the critical two-phase transport issues in microchannel flow boiling, will be proposed including “two-phase oscillator” and “two-phase separation”. New designs based on the two concepts are explored to significantly enhance flow boiling by promoting nucleate boiling, convection and thin film evaporation through controlling two-phase transport. For example, multiple micronozzles are designed to efficiently remove confined bubble and extend mixing in microchannel by creating “two-phase oscillator” to generate high frequency jetting flows and high frequency two-phase oscillations. Furthermore, to achieve a better control of two-phase flow patterns, i.e., realizing highly desirable annular flow, novel enhanced-capillary-pressure configurations have been developed to achieve “two-phase separation” through rectifying stochastic liquid/vapor interfaces into on-demand manner.


Enhanced Flow Boiling in Microchannels by Integrating Multiple Micronozzles and Reentry Cavities on HFE-7100

Enhanced flow boiling in microchannels by self-sustained high frequency two-phase oscillations

Related Publication:

W.M. Li, F.H. Yang, T. Alam, X.P. Qu,, Wei Chang, and Chen Li, “Enhanced Flow Boiling in Microchannels using Auxiliary Channels and Multiple Micronozzles (I): Characterizations of Flow Boiling Heat Transfer,” Int. J. of Heat and Mass Transfer, 116 (2018) 208-217.


W.M. Li, F.H. Yang, T. Alam, X.P. Qu,, Wei Chang, and Chen Li, “Enhanced Flow Boiling in Microchannels using Auxiliary Channels and Multiple Micronozzles (II): Enhanced CHF and Reduced Pressure Drop,” Int. J. of Heat and Mass Transfer, 115 (2017) 264-272.


W.M. Li, X.P. Qu, T. Alam, Wei Chang, and Chen Li, “Enhanced Flow Boiling in Microchannels through Integrating Multiple Micro-nozzles and Reentry Microcavities,” Applied Physics Letters. 110 (2017), 014104


W.M. Li, F.H Yang, T. Alam, J. Khan, Chen Li, "Experimental and theoretical studies of critical heat flux of flow boiling in microchannels with microbubble-excited high-frequency two-phase oscillations", International Journal of Heat and Mass Transfer Vol. 88, pp. 368–378. 2015.


Fanghao Yang, Xianming Dai, Chih-Jung Kuo, Yoav Peles, Jamil A. Khan, and Chen Li, “Enhanced Flow Boiling in Microchannels by Self-sustained High Frequency Two-phase Oscillations,” International Journal of Heat and Mass Transfer, Vol. 58, pp. 402-412, 2013.


F.H. Yang, M. Alwazzan, W. Li, Chen Li, "Single-and Two-Phase Thermal Transport in Microchannels With Embedded Staggered Herringbone Mixers", Journal of Microelectromechanical System, Vol. 23, pp. 1346-1358, 2014.


F.H. Yang, X.M. Dai, Y. Peles, P. Cheng, J. Khan, and Chen Li, “Flow Boiling Phenomena in a Single Annular Flow Regime in Microchannels (I): Characterization of Flow Boiling”, International Journal of Heat and Mass Transfer, vol. 68(0), pp. 703-715, 2014.




Related Posters

Wenming Li, Tamanna Alam, Wei Chang, Jamil Khan and Chen Li, Enhanced flow boiling with HFE7100 in microchannels coupling multiple nozzles with reentry cavities.


Fanghao Yang, Xianming Dai and Chen Li, Enhanced Flow Boiling in Microchannels by Self-sustained High Frequency Two-phase Oscillations.


3- Thermal shock study in microchannel flow boiling

Thermal shock is common in real electronics components and the main reason to cause the function failure especially for components with large power density. Flow boiling in microchannel can bring out more heat due to phase change of working medium compared to traditional conduction or heat pipe techniques. However, thermal shock in the microchannel has not yet been fully studied. The aim of this study is to characterize the thermal shock effects on current microchannel design under certain working conditions and explore the feasibility of microchannel cooling technique for electronic cooling applications.

Schematic of transient flow boiling set-up for thermal shock study

4- Explore a Unified Ultra-Efficient and Gravity-Insensitive Flow Boiling pattern for Space Applications

A novel boiling surface with engineered submicron pores (formed by NW bundles) surrounded by nanoscale pores (created by individual NWs) were developed earlier by our team.

SiNW enables gravity-insensitive bubble departure mechanism (enhances bubble nucleation site density and departure frequency; reduces bubble departure diameter).

Regulate flow regime development (Reduced the transitional flow boiling regimes (slug/churn/ wavy) to a single annular flow).

Significant progress has been made to understand and characterize the flow boiling behaviors in SiNW microchannels using water and dielectric fluids experimentally and visually in our studies. Enhanced heat transfer performances with extended critical heat flux limit, reduced flow boiling instabilities and pressure drop and excellent orientation independency have been observed. SiNW introduces explosive boiling, reduces intermittent flow regimes (slug/ churn), improves rewetting, maintains thin film and thus, helps to improve system performances.

SEM images of SiNW microchannel surface

Reduced bubble diameter and Transformation from bubbly flow to annular flow in SiNW microchannels

Nanowire Microchannels



Related Publication:

T. Alam, W.M. Li, F.H. Yang, W. Chang, J. Li, Z. Wang, J. Khan and Chen Li, “Force Analysis and Bubble Dynamics during Flow Boiling in Silicon Nanowire Microchannels,”  Int. J. of Heat and Mass Transfer, 101 (2016) 915-926.


T. Alam, A.S. Khan, W.M. Li, F.H. Yang, Y. Tong, J. Khan and Chen Li, “Transient Force Analysis and Bubble Dynamics during Flow Boiling in Silicon Nanowire Microchannels,”  Int. J. of Heat and Mass Transfer, 101 (2016) 937-947.



Related Posters

Tamanna Alam, Wenming Li, Fanghao Yang, Jamil Khan, Chen Li, Orientation Effects on Flow Boiling Silicon Nanowire Microchannels.




Pool Boiling

1- Pool boiling

The pool boiling experimental setup allows testing flat surface areas that can reach up to 2cm×2cm under saturation conditions of the atmosphere. This setup capable to reach high heat flux ranges to examine the heat transfer performance within wider ranges of subcooling, and to assure reaching the critical heat flux. Three windows are installed on the testing chamber allowing suitable visualization access to record and monitor the bubble dynamic during the boiling process.

Bubble Dynamic-Experiment

2- Numerical simulation of phase change phenomenon - Pool boiling

Our goal is to develop a comprehensive model to simulate the phase change phenomena, to reach this goal OpenFOAM software has been employed and modified. We capture two phase characteristics of phase change phenomena by employing a volume of fluid (VOF) algorithm which is solved is addition to a PISO pressure-velocity coupling algorithm and a coupled matrix for energy equation. Our model is able to capture the interfacial phase change using a formulation based on kinetic theory, effects of micro-layer formed under the bubble adjacent to the wall is included by assuming a linear distribution of micro-layer in computational cells adjacent to the wall and eventually heater effects are also included by solving energy equation in the heater and employing a heat continuity boundary and solid, fluid interface.

Our code is being examined against available experimental, numerical and analytic investigations in literature.

Right now we are finalizing the code and simulating pool boiling with homogeneous nucleation model, to later on move to simulation of boiling phenomena in confined domains and micro-channels.

Bubble Dynamic-Simulation



1- Numerical Analysis of Phase Change, Heat Transfer and Fluid Flow within Miniature Heat Pipes

Computation of flow and heat transfer in a heat pipe is complicated by the strong coupling among the velocity, pressure and temperature fields with phase change at the interface between the vapor and wick. Not to mention, the small size and high aspect ratio of heat pipes brings their own challenges to the table. In this dissertation, a robust numerical scheme is employed and developed to investigate transient and steady-state operation of cylindrical heat pipes with hybrid wick structure for high heat fluxes based on an incompressible flow model. Despite many existing works, this is accomplished assuming as few assumptions as possible. The fundamental formulation of heat pipe is developed in such a way to properly take into account the change in the system pressure based on mass depletion\addition in the vapor core. The numerical sensitivity of the solution procedure on phase change at the liquid-vapor interface are recognized and effectively handled by reformulating the mathematical equations governing the phase change. Hybrid wick structure of the heat pipe is modeled accurately to further investigate thermal and vicious novel wick structures. A fully implicit, axisymmetric sequential finite volume method is devised in conjunction with the SIMPLE algorithm to solve the governing equations. ANSYS Fluent software with the power of User Defined Functions and User Defined Scalars is used to apply the numerical procedure in coupled system and standard levels. This two-dimensional simulation can solve for symmetrical cylindrical and flat heat pipes, and also can be simply developed to solve for three-dimensional flat and non-symmetrical cylindrical heat pipes.


Related Publication:

M. Famouri, G. Carbajal, and Chen Li, “Transient Analysis of Heat Transfer and Fluid Flows in a Polymer-based Micro Flat Heat Pipe with Hybrid Wicks,”  International Journal of Heat and Mass Transfer, Vol. 70, pp. 545-555, 2014.



Related Poster:

M. Famouri, G. Carbajal, and Chen Li, Transient Analysis of Heat Transfer and Fluid Flow in a Polymer-based Micro Flat Heat Pipe with Hybrid Wicks.


Temperature distribution in heatpipe

Temperature distribution in vapor core

Velocity distribution in wick

2- High performance heat pipe with enhanced wick structure

Nowadays, heat pipe is an effective heat transfer component for heat dissipation and thermal management such as IC cooling, solar energy collector and thermal power station. The thermal performance of heat pipe is greatly influenced by wick structure. Hybrid wick has the potential to enhance the thermal performance of heat pipe by balancing permeability and capillary force. We study on a groove-mesh hybrid wick, which is fabricated in round copper heat pipe. The purpose of the designed is to achieve the best effective thermal conductivity of heat pipe by enhancing the evaporation.


Heatpipe with fins


Three wick structures




1- Long Term Experiments of Dropwise Condensation Coatings

Dropwise condensation is an expected heat transfer in industrial applications due to its more than 10 times higher heat transfer coefficient than that of filmwise condensation. However, the durability of the coatings for enhancing dropwise condensation inhibits their applications. Therefore, a closed test set-up is designed and fabricated which can be operated at atmospheric pressure and vacuum, mainly for developing a high fidelity equations or correlations to predict the degradation rate of the coatings such as graphene and the self-assembled monolayers for dropwise condensation as well as explaining the mechanism of the durability of the coatings.

The designed for long term dropwise condensation

The fabricated test set-up for long term dropwise condensation

2- Condensation

This setup allows for condensation experiments to be conducted on tube and flat configurations under saturation conditions of the atmospheric pressure. Various methods are adapted on the condensing surface to enhance the condensation heat transfer performances. This includes methods that only target reducing the surface energy on the condensing surface to obtain dropwise condensation mode since it is significantly higher than the filmwise condensation mode which is the most common mode in industrial and daily individual applications. Other methods are also adapted to enhance the condensation heat transfer performance by combining two wettability regions on the condensation surface leading to a hybrid condensation mode. These regions of different wettability can take variety of configurations and scales. The ultimate target for all approaches is to expedite the droplets removal mechanism on the condensing surface allowing for higher condensation rate.


Related paper

M. Alwazzan, K. Egab, B.L. Peng, J. Khan, and Chen Li, “Condensation on hybrid/patterned copper tubes (I): characterization of condensation heat transfer,” Int. J. of Heat and Mass Transfer, 112 (2017) 991-1004.


M. Alwazzan, K. Egab, B.L. Peng, J. Khan, and Chen li, “Condensation on hybrid/patterned copper tubes (II): visualization study of droplet dynamics,” Int. J. of Heat and Mass Transfer, 112 (2017) 950-958.


3- Condensation

The second condensation testing setup allows condensation experiments to be conducted in the same manner of the first setup, but in a close loop instead. This allows to experimentally test the durability of the enhancement method under variety of saturation conditions, such as under higher lower pressure than the atmosphere. In addition, since it is a close loop setup, it allows continuous condensation testing for prolong period of times.




1- Sweating-boost air cooling on nanoscale wick structures

Low heat transfer coefficient (HTC) in air/fin-side is the bottleneck of dry cooling strategies for thermal power plants. Inspired by the phase change heat transfer during the perspiration of mammals, a sweating-boosted air cooling strategy with on-demand water dripping is proposed. The testing samples are featured with macroscale grooves for global liquid delivery, and with nanoscale hydrophilic copper oxide (CuO) wick structures for local liquid spreading. The experiments of sweating-boosted air cooling are conducted in a wind tunnel system. There are three wetting conditions with increasing dripping rates: dry, partial wetted, and flooded conditions. In the partial wetted conditions, the surface temperatures reduce and HTCs increase with increasing dripping rates. For a given dripping rate of water, HTCs are enhanced and surface temperatures are reduced with increasing air velocities. High air velocity and low surface temperature have a trade-off effect on the evaporation process. This effect results in almost constant saturated dripping rates for a given thermal load. There is a linear relationship between the saturated dripping rates and the thermal loads. It confirms that the evaporation dominates the heat transfer process of sweating-boosted air cooling. Complete surface wetting is obtained on the designed surfaces, but no obvious effect of groove width on HTCs is observed. Sweating-boosted air cooling can significantly increase air-fin side HTC in air cooled condenser (ACC), and dramatically reduce the water consumption compared to current water evaporative condenser (WEC). This research provides the fundamental understanding on the sweating-boosted effects on the air cooling.


Schematic of sweating-boosted air cooling strategy


Experimental setup for the sweating-boosted air cooling


Testing sample with grooves (a) before and (b) after hot alkaline treatment (W = 0.75 mm). CuO nanostructures characterized with FE-SEM (c) and (d) AFM.


Typical thermal performance of the sweating-boosted air cooling. The solid and hollow symbols indicate the surface temperature and the HTC, respectively.


Related paper

P.T. Wang, R. Dawas, M. Alwazzan, W. Chang, J. Khan, and Chen Li, “Sweating-boosted air cooling on nanoscale CuO wick structures,” Int. J. of Heat and Mass Transfer, 111(2017)817-826.



Phase Change Material

1- The Charging/Discharging Performances of Phase Change Materials

Phase change materials have been extensively applied in the thermal management of heat storage system and the buildings. Choosing an appropriate phase change material with good charging/discharging performances (stable temperature distribution during charging/discharging, short charging/discharging time, high charging/discharging rate and high charging efficiency) is extremely significant. As a result, an experimental system is designed and built, aiming to investigating the charging/discharging performances of different kinds of phase change materials such as paraffin wax and eutectic (metal alloys) with low melting point thereby providing a reasonable guideline for choosing an effective phase change material for the specific applications.

Schematic diagram of the experimental system for the charging/discharging performance of phase change materials