Constant Temperature Boundary Research Articles

Heat pipe failure accident analysis in megawatt heat pipe cooled reactor

The heat pipe cooled reactor is an advanced reactor design which is nearly solid-state with an array of in-core heat pipes for passive heat transfer. The local heat pipe failure probability is certainly high over the reactor lifetime. Therefore, the reactor design must include consideration of heat pipe failure accidents which needs to be explored in-depth. The Monte Carlo neutron transport code RMC was used with the general finite element analysis software ANSYS Mechanical to analyze the thermal/mechanical characteristics of a megawatt heat pipe cooled reactor for both normal and heat pipe failure conditions. The simulations were verified against a previous study with the predicted temperature distributions being consistent with each other with an absolute error of less than 3 K. Further analyses show that a 2-D model simulation can also well represent the 3-D model for heat pipe failure accident analyses. Sterbentz et al. (2017) simplified the heat pipe model with a constant temperature boundary condition along the evaporator wall of 950.15 K regardless of the heat load or heat pipe operating temperature. Therefore, a heat pipe subroutine was added to the codes to predict the heat pipe temperatures during a heat pipe failure accident. The results show that the heat pipe failure will cause larger temperature rises and stress concentrations. The heat pipe wall temperatures in the core differ from each other with the differences between the heat pipe loads during normal heat pipe operation and with failures reflecting the local effect of the heat pipe failures. The operating conditions far from the failure area are little affected (<20 K temperature change) by a single heat pipe failure. The effects of the heat pipe failure on the reactor core reactivity and local power distribution were also analyzed with a reactivity change of less than 5 pcm and a local power change of less than 0.1%. Therefore, the heat pipe failure can be analyzed in steady-state simulations with unchanged power distributions.

Natural convection of viscoplastic fluids in an enclosure with partially heated bottom wall

In this paper, heat transfer characteristics and flow patterns of viscoplastic fluid due to natural convection in a two-dimensional heated enclosure have been investigated both computationally and experimentally. The enclosure is heated partially at the bottom wall at its centre. The side walls are isothermally cooled. The rheology of the enclosed fluid is represented by Bingham model. In addition, an experimental investigation on the viscoplastic test fluids, prepared from Carbopol Ultrez 20 polymer, has also been carried out. The proposed computational model has been validated with our own experimental results. The results in terms of flow and thermal characteristics are presented for different values of Bingham number (Bn) varying between 0 and 30, Rayleigh number (Ra) ranging 104 to 106, Prandtl number (Pr) from 1 to 100 and heated lengths of the bottom wall (ε). The critical value of yield stress of enclosed fluid for which convection circulation diminishes and conduction becomes the sole mode of heat transfer in the enclosure has also been evaluated. The effect of constant heat flux boundary condition at the heated portion has been compared with that of constant temperature (isothermal) boundary conditions. It has been found that convection is weak in constant heat flux case compared to the isothermal condition case.

Effectively infinite thermal conductivity and zero-index thermal cloak

Thermal conductivities are the key to heat transfer, and high thermal conductivities are generally beneficial and sometimes indispensable to design thermal metadevices. Therefore, the exploration of high thermal conductivities never stops. However, infinite thermal conductivities have not yet been discovered. To solve the problem, we propose an exact approach to effectively infinite thermal conductivities with a constant-temperature boundary condition, which can be easily realized by an external thermostatic sink. Since (effectively) infinite thermal conductivities just correspond to zero refractive indexes in photonics, they have direct applications in designing zero-index thermal metadevices. For example, we experimentally demonstrate zero-index thermal cloaks which can work in highly conductive backgrounds with simple structures. These results provide insights into thermal metadevices and thermal management with effectively infinite thermal conductivities.

Flow and Thermal Transport Through Unit Cell Topologies of Cubic and Octahedron Families

Advancements in Additive Manufacturing (AM) routes have opened several opportunities for manufacturing complex porous structures which could not have been manufactured by the conventional foaming process that essentially results in unit cells represented as Tetrakaidecahedron (TKD) shape. The regular open-cell lattices belonging to the cubic and octahedron family of structures provide wide spectrum of porosity, pore-density, permeability and surface area-to-volume ratios. These complex cell topologies can potentially be manufactured additively, making the realization of above-mentioned properties possible. For efficient heat exchanger designs, the reticulated structures made from these unit cells are expected to have higher permeability, porosity, effective thermal conductivity and interstitial heat transfer coefficient. A detailed understanding of local flow and heat transport characteristics is necessary to design unit cells with such properties. To this end, four different metal foam unit cell topologies have been investigated in the present study, viz. Tetrakaidecahedron (TKD), Cube, FD-Cube and Octet. The Cube and FD-Cube unit cell topologies investigated in the present study belong to the cubic family, where the fibers are present only along the edges and face of the unit cell volume. The TKD and Octet configurations belong to the octahedron class of lattices, where complex connectivity exists within the unit cell volume. To allow for direct comparison between these three unit cells, circular fibers are chosen, which is different from the general TKD cells (tricuspid cross-sections). The porosity was fixed 0.986 which dictated the fiber dimensions of respective unit cells, with Octet fibers being thinnest. Direct simulations have been carried out with periodic boundary conditions on unit cell faces which were parallel to the flow. Such configurations when reticulated in three dimensions are representative of porous structures which are single-cell thick in the streamwise direction. These foam shapes have been proven to be very efficient as they do not allow the thermal boundary layer development. Flow and heat transfer results obtained for above boundary conditions have also been compared with the periodic boundary condition on all the faces.The direct simulation results include pressure drop per unit length and volumetric Nusselt number (corresponding to constant heat flux and constant wall temperature boundary conditions on fibers). Flow stagnation on fibers was the main contributor for both, pressure drop increase, and interstitial heat transfer enhancement. TKD unit cell had higher flow losses and thermal performance than FD-Cube. Octet (octahedron family) configuration had the highest flow losses as well as Nusselt number compared to all other unit cell topologies, because of the large surface area offered for flow stagnation.

Experimental and Numerical Analysis of Additively Manufactured Inconel 718 Coupons With Lattice Structure

Abstract In the present paper, two lattice geometries suitable for near surface and double wall cooling were developed and tested. The first type of unit cell consisted of six ligaments of 0.5 mm diameter joined at a common vertex near the middle. The second type of unit cell was derived from the first type by adding four mutually perpendicular ligaments in the middle plane. Two lattice configurations, referred to as L1 and L2, respectively, were obtained by repeating the corresponding unit cell in streamwise and spanwise directions in an inline fashion. Test coupons consisting of these lattice geometries embedded inside rectangular cooling channel with dimensions of 2.54 mm height, 38.07 mm width, and 38.1 mm in length were fabricated using Inconel 718 powder and selective laser sintering (SLS) process. The heat transfer and pressure drop performance was then evaluated using steady-state tests with constant wall temperature boundary condition and for channel Reynolds number ranging from 2800 to 15,000. The lattices depicted a higher heat transfer compared with a smooth channel and both the heat transfer and pressure drop increased with a decrease in the porosity from L1 to L2. Steady-state conjugate numerical results revealed formation of prominent vortical structures in the inter-unit cell spaces, which diverted the flow toward the top end wall and created an asymmetric heat transfer between the two end walls. In conclusion, these lattice structures provided an augmented heat transfer while favorably redistributing the coolant within channel.

Impact of convective cooling on pore pressure and stresses around a borehole subjected to a constant flux: Implications for hydraulic tests in an enhanced geothermal system reservoir

Hydraulic tests are commonly performed prior to reservoir stimulation to determine the natural permeability of the enhanced geothermal system (EGS), which typically involves injection of cold water at low rates into the hot target EGS reservoir and measurement of the surface pressure, flow rate, and thermal drawdown simultaneously. Interpretation and analysis of the data collected during a hydraulic test are largely based on fitting type curves generated from analytical solutions. We have formulated a geomechanical model for hydraulic tests in a thermoporoelastic EGS reservoir in which the open-hole section of the borehole wall is subjected to a constant-rate flux and convective cooling. We derive the analytical solutions for injection induced axisymmetric deformation, temperature drawdown, pore pressure, and stresses in the Laplace domain, and we obtain time-domain solutions using the Stehfest inversion algorithm. We evaluate numerical examples to illustrate the competition between the thermoelastic effect and the poroelastic effect in controlling temperature, pore pressure, and stresses around the borehole wall. The numerical results indicate that temperature drawdown at the borehole wall would be significantly overestimated if a constant temperature boundary condition instead of convective cooling is used. At early times, injection induced hoop stress/pore pressure is partially offset by the corresponding ones induced by convective cooling. At late times, convective cooling plays a marginal role in influencing pore pressure and stress around the borehole. The analytical solutions are helpful to address the thermohydromechanical coupling mechanisms controlling pressure perturbations during hydraulic tests conducted in an EGS reservoir.

Performance of annular thermoelectric couples by simultaneously considering interface layers and boundary conditions

Annular thermoelectric couple (ATEC) is advantageous in eliminating the contacting thermal resistance between the flat-plate thermoelectric couple and the round-shaped heat source/heat sink, which can maximize the energy utilization. For a more precise performance prediction, a theoretical ATEC model is established with consideration of the interface layers, which adopts the boundary conditions of constant heat flux (Qin) on the hot side and convection on the cold side. Boundary condition of heat flux can match many applications of waste energy recovery and low-grade energy utilization, and convection boundary condition can match many practical cooling systems as well. New analytical solutions for the temperature field and performance are obtained in this work. The results show that the variation trend of performance with the annular shape parameter Sr is entirely different between under constant temperature boundary conditions and under boundary conditions of heat flux and convection. Impedance of interface layers causes temperature profile reduction and performance degradation, especially in relatively short ATECs. Increasing the length of thermoelectric leg can improve the performance of ATECs and diminish effects of interface layers. This study is beneficial for designing the actual annular thermoelectric generators.

Performance comparison between rectangular and trapezoidal-shaped thermoelectric legs manufactured by a dispenser printing technique

At present, device engineering has been limited to the rectangular-shaped TE leg. Therefore, a trapezoidal-shaped leg has been proposed for the TE system and prototypes are developed in this work. Performance comparison has been investigated between rectangular and proposed trapezoidal-shaped leg based TE prototypes. The n-type (0.98Bi,0.02Sb)2(0.9Te,0.1Se)3 and p-type (0.25Bi,0.75Sb)2(0.95Te,0.05Se)3 are considered as base material with Durabond-950 binder material to manufacture TE legs by using a cost-effective dispenser printing technology. The current study includes analysis of SEM imaging, characterization of manufactured TE legs, various experimental tests on TE prototypes, comparison between analytical and experimental results, and cost analyses. For the given restricted volume envelope, the trapezoidal-shaped TE prototype generates 1.24 times more voltage and 1.5 times more power when compared to the rectangular-shaped prototype at 30 °C hot side temperature when the cold side is exposed to the surrounding. For a given constant temperature boundary conditions (i.e., ΔT = 10 °C), the rectangular-shaped TE prototype harvests 1.4 times more power than the trapezoidal-shaped one, while the power density for rectangular TE prototype (i.e., 0.37 W/m3) is almost the same as trapezoidal one (i.e., 0.36 W/m3). Furthermore, the proposed trapezoidal-shaped prototype uses 28.6% less material by mass than the rectangular prototype.

Anomalous enhancement of convective heat transfer with dispersed SiO2 particles in ethylene glycol/water nanofluid

The heat transfer coefficients and friction factor for SiO2 – ethylene glycol/water (50 wt%) nanofluids, with the SiO2 particle diameter ranging from 100 to 1000 nm, and particle weight concentration ranging up to 10%, were measured for flow in a circular tube under constant temperature boundary conditions. The measurements were performed under turbulent flow conditions, wherein the temperature at the entrance of the test section was 353.15 K. Both the heat transfer coefficient and friction factor of the SiO2 nanofluids are higher than those of the ethylene glycol/water system. The heat transfer coefficient of the SiO2 nanofluids increases with particle concentration up to 7 wt% and saturates thereafter. Interestingly, the heat transfer coefficient of the SiO2 nanofluids depends on the particle diameter and it attains maximum value at the diameter of 300 nm and solid fraction of 7 wt%. The heat transfer coefficient of SiO2 nanofluids increases by up to 25% as compared with that of the ethylene glycol/water solution. Additionally, a new correlation formula is proposed to correlate the Nusselt numbers and the Reynolds numbers for the SiO2 nanofluids.

Modeling of shallow, horizontal, unsaturated, ground-based heat exchangers with consideration of dry zone formation

High fluid temperatures in ground-based heat exchangers (GHXs) during the cooling season may result in a decrease in thermal conductivity of adjacent backfill (λbackfill), potentially causing degradation in the performance efficiency of the GHX system. In this study, numerical modeling was performed using the SVOffice™ finite-element software to evaluate coupled heat and moisture transfer around two GHXs. Constant-temperature boundary conditions of 35 ºC were applied to the GHX surfaces, and thermal properties of the backfill were controlled for comparisons. For estimate typical hydraulic and thermal properties for the modeling, laboratory tests and literature review were performed. Modeling results indicate that coupled heat and moisture transfer occurs rapidly near the GHX involving a dry zone formation when λbackfill decreases. A boundary between dry and wet zones where soil thermal properties rapidly change was observed around 50% GHX temperature dissipated (T50), and accordingly T50 was used to optimize the pipe configuration. Coupled heat and moisture transfer increased when the GHX configurations were optimized with consideration of dry zone formation. These results imply that thermally enhanced, engineered backfill and optimized configurations can enhance GHX system efficiency.

The development of a three-dimensional finite element method code for the heat flux analysis of tungsten monoblock divertor on EAST

The calculation of heat flux on the divertor is critical to study the characteristics of energy transport in controlled fusion plasma. Since 2014, the upper divertor of Experimental Advanced Superconducting Tokamak (EAST) has been upgraded into water-cooled W/Cu monoblock, and therefore the original two-dimensional(2D) heat calculation code does not apply to the heat flux calculation on the upper divertor due to the asymmetry toroidal geometry. Thus, it is urgent to develop a Three-dimension(3D) heat flux calculation code based on Finite Element Method(FEM) to determine the 2D heat flux distribution at the divertor surface in EAST. This paper presents a detailed design and development of this 3D FEM heat flux calculation method with the full consideration the monoblock geometry of the upper divertor. Based on the surface temperature captured from the IR camera, and the effect of a thin surface layer on the divertor, the heat transmission coefficient was determined in the solution of heat conduction equation. To benchmark this code, the calculation results are compared with the results using ANSYS software with the same conditions(constant temperature boundary and constant heat flux boundary). The comparison results show good agreements that the maximum fractional error between the two methods is less than 4 %. Finally, the characteristics of heat flux distribution on the divertor due to edge localized modes(ELMs) losses on EAST are also demonstrated.

Conjugate forced convective heat transfer in a sandwich panel with a Kagome truss core: The effects of strut length and diameter

Heat exchangers with a cellular metal core have become an alternative to those with louvered fins due to their high heat transfer performance and mechanical strength. Because of the multi-functionality, they are mostly used in systems such as rockets and space vehicles or in others in need of both high heat transfer performance and mechanical strength. The geometrical parameters of the unit cell of cellular metals have important effects on both fluid and heat transfer characteristics and these effects need to be studied. Accordingly, conjugate forced convective heat transfer in a sandwich panel with a Kagome truss core both with a constant heat flux (CHF) and a constant temperature (CT) boundary condition was studied numerically in this study for various values of the Reynolds number, strut length, and strut diameter. The 3D turbulent flow and heat transfer was modeled by the RNG k-epsilon turbulence model. The near-wall behavior of the flow was modelled by enhanced wall treatment. The channel was extended before and after the Kagome core to avoid entrance and exit effects. The computational results were obtained for the Reynolds numbers in the range of about 6000–17000, for the strut lengths of 3, 5 and 7 mm, and for the strut diameters of 1.3 and 1.6 mm. A nonlinear regression analysis was also performed for the pressure loss coefficient and average Nusselt number. The numerical results show that complex flow structures develop in the flow field due to the presence of unit cells. The results also show that the pressure drop along the Kagome core increases considerably with decreasing strut length and increasing strut diameter. The average Nusselt number increases significantly when the strut length is decreased and shows a small increase with an increase in the strut diameter. The effect of strut diameter on average Nusselt number becomes marginal for strut length of 3 mm. The average heat transfer rate for a fixed pumping power shows a considerable increase with a decrease in the strut length and a small increase with the strut diameter for high values of strut length. On the other hand, for the strut length of 3 mm, it shows a small decrease with an increase in the strut diameter for the CT case and remains almost constant for the CHF case.

Numerical Study of Laminar Flow Separation in Step Geometry Using Corners Formulation in Lattice Boltzmann Method

In this study, the velocity and temperature distributions of incompressible laminar flow of a step downstream have been investigated using the lattice Boltzmann method. The inlet and outlet flows at internal and external corners of this step have been simulated using the single relaxation time model in various Reynolds number and expansions ratios, two dimensionally. According to Zou and He, constant temperature boundary conditions and open-boundary condition have been utilized at inlet and outlet, respectively. The model results have been validated by experimental data. It was obtained that there are agreements between them. It was resulted that the length of recirculation zone is a function of Reynolds number of inlet flow. Increasing the Prandtl number decays the rate of fluid temperature decrement. Furthermore, in constant Reynolds and Prandtl numbers, decreasing the expansion ratio decreases the Nusselt number of flow. Generally, the ability of lattice Boltzmann method in simulating complex fluid and thermal flows has been shown.

Integral Solution of Two-Region Solid–Liquid Phase Change in Annular Geometries and Application to Phase Change Materials–Air Heat Exchangers

A mathematical model based on the integral method is developed to solve the problem of conduction-controlled solid–liquid phase change in annular geometries with temperature gradients in both phases. The inner and outer boundaries of the annulus were subject to convective, constant temperature or adiabatic boundary conditions. The developed model was validated by comparison with control volume-based computational results using the temperature-transforming phase change model, and an excellent agreement was achieved. The model was used to conduct parametric studies on the effect of annuli geometry, thermophysical properties of the phase change materials (PCM), and thermal boundary conditions on the dynamics of phase change. For an initially liquid PCM, it was found that increasing the radii ratio increased the total solidification time. Also, increasing the Biot number at the cooled (heated) boundary and Stefan number of the solid (liquid) PCM, decreased (increased) the solidification time and resulted in a greater (smaller) solid volume fraction at steady state. The application of the developed method was demonstrated by design and analysis of a PCM–air heat exchanger for HVAC systems. The model can also be easily employed for design and optimization of annular PCM systems for all associated applications in a fraction of time needed for computational simulations.

Heat transfer investigation of combined electroosmotic/pressure driven nanofluid flow in a microchannel: Effect of heterogeneous surface potential and slip boundary condition

Abstract In the present study, electroosmotic and pressure driven Newtonian nanofluid flow in a microchannel with constant temperature boundary condition is studied via Lattice Poisson–Boltzmann method. In order to validate the numerical solution, the computed results are compared with some existing analytical solutions. Then, effects of different parameters such as ratio of pressure velocity to Helmholtz–Smolochowski velocity, slip coefficient, nanoparticles volume fraction and diameter on flow field and heat transfer is examined. The results show that by fixing the electric field and increasing the pressure force, Nusselt number decreases, and it increases by fixing the pressure force and increasing the electric field. Also, increasing the slip coefficient causes the velocity enhancement in the electroosmotic flow. Furthermore, increasing the nanoparticles volume fraction decreases the velocity and increases Nusselt number. Finally heterogeneous surface potential moves the vortices toward the walls and enables controlling the quantity and direction of velocity field.

Integrating thermal‐concentration smoothed profile with lattice Boltzmann methods for simulating sedimentation of nonisothermal circular particles

SummaryA thermal‐concentration smoothed profile‐lattice Boltzmann method is proposed to study the effect of the concentration field on the dynamic behavior of nonisothermal cylindrical particles during the sedimentation process. The velocity, temperature, and concentration equations are solved using the lattice Boltzmann method. Moreover, the smoothed profile method is employed to enforce the nonslip boundary condition as well as constant temperature and constant concentration boundary conditions at the particles surfaces. Moreover, the Boussinesq approximation is used to couple the velocities, temperatures, and concentrations fields. The proposed combined method is validated by comparing the present numerical results with those found in the literature, showing good consistency. Then, the effect of the concentration buoyancy on the behavior of nonisothermal particles is discussed. In addition, the effect of Prandtl, Schmidt, and thermal Grashof numbers on the settling process is investigated. The results show that, by adding the effect of concentration, the maximum settling velocity of hot particles is reduced more relative to the cold ones; accordingly, the cold particles are settled faster than the hot ones. Finally, the sedimentation of two particles in a container at high thermal Grashof is investigated. It is shown that, at high thermal Grashof, there is an intense competition between the buoyancy force and gravity for the hot particles. The buoyancy flow generated leads to the reversal of the drafting‐kissing‐tumbling motion of the hot particles, making the particles move upward.

Study of the effect of tilting lateral surface angle and operating parameters on the performance of a vertical shell-and-tube latent heat energy storage system

In this paper, a comprehensive mathematical model is developed to study the thermal energy storage performance in vertical shell-and-tube latent heat thermal energy storage (LHTES) systems with different tilting lateral surface angles. This model integrates heat transfer in heat transfer fluid (HTF), HTF tube and phase change materials (PCM). It is validated using experimental data and then applied to investigate effects of tilting lateral surface angles on the enhancement of natural convection in the LHTES system. The results show that increasing the tilting lateral surface angle largely enhances heat transfer in the melting process whilst it has negative effects on the solidification process. The total melting time reduces by 45% as the tilting angle increases from 0° to 7°. An optimal tilting angle of 4° is identified based on the trade-off between melting and solidification processes. Furthermore, the effect of HTF operating parameters on the performance of the LHTES system is evaluated. It shows that the melting and solidification time of the LHTES system with a tilting angle 4° reduces by 22% and 8.5%, respectively, as the HTF flow Reynolds number increases in laminar and transition flow regions. However, increasing the HTF flow Reynolds number does not show significants effect under turbulent flow regions. The variation of the HTF outlet temperature is also investigated. The temperature difference between the HTF inlet and outlet is found small at all studied HTF flow rates, which indicates that assumption of the constant HTF surface temperature boundary in simulations in the literature is reasonable.

Measurement of Thermophysical Properties of Loose Coal Based on Multi-dimensional Constant Temperature Boundary Unsteady Heat Transfer Model

Based on the unsteady state heat transfer model, production solution method is proposed to calculate the thermal diffusivity and thermal conductivity of loose coal. By constructing the heat transfer model of the infinite large plate and the infinitely long cylinder, the temperature rise formula of the production solution method is established. The measured temperature rise is then used by the MATLAB programming software to inversely calculate the thermophysical parameters of the loose coal. During the experiment, a constant temperature boundary was constructed using an incubator and a high thermal conductivity copper cylindrical case. The experiment selected anthracite, bituminous coal in Panyi Mine, and bituminous coal in Lizuo Mine for thermophysical property measurement. The results show that the relative deviation between each measurement result and related literature is within 5 %, and each sample is tested three times. The deviations are less than 5 %, which verify the stability of the test method. Finally, the experimental model was established in the ANSYS FLUENT simulation software, and the thermophysical parameters of the measured coal samples were substituted into the model for simulation calculation. It was found that the simulation temperature rise was consistent with the measured temperature rise.

Effect of thermal boundary condition on forced convection from circular cylinders

Researchers commonly assume that the effect of constant temperature and constant heat flux thermal boundary condition on forced convection from a circular cylinder is negligible. Such claim was assessed using Computational Fluid Dynamics for Reynolds number, Re, ≤ 3,000. The flow was directly solved without modeling for Re ≤ 300, and Large Eddy Simulation was used for Re = 1,000 and Re = 3,000. Constant property simulations were done for Prandtl number of 0.7. Fluid dynamics parameters: drag coefficient, Strouhal number, separation angle and recirculation length, were used to assess the accuracy of the simulations. The main parameter of interest, the Nusselt number, is higher locally for constant heat flux than constant temperature for 60 ≤ Re ≤ 3,000. In addition, the ratio of overall Nusselt number for constant heat flux to that for constant temperature increases for Re < 30 and is ∼1.15 between 30 ≤ Re ≤ 3,000, showing that thermal boundary condition has significant effect on heat transfer from circular cylinder. Finally, the study developed empirical correlations relating overall Nusselt number to studied Reynolds number range.

Numerical investigation on the heat transfer characteristics of liquid-liquid plug-train in microchannels

Liquid-liquid plug-train is of great importance due to its wide applications in the heat transfer enhancement and the richness of underlying physics. This paper presented a theoretical and numerical study of the liquid-liquid plug-train in a microchannel. Based on the analytical model of flow field, the heat transfer process in liquid-liquid plug-train was investigated. The constant surface temperature boundary condition was considered. Nusselt number (Nu) and heat transfer index (η) were employed to evaluate the heat transfer characteristics of liquid-liquid plug-train in the microchannel. Three stages of the heat transfer process in liquid-liquid plug-train were identified: (i) development of thermal boundary layer (TBL); (ii) advection of heated/fresh fluid in the liquid-liquid plug train; and (iii) thermally fully developed flow. The effects of the thermal conductivity ratio and the plug length ratio were investigated. A low plug length ratio was found to be effective in heat transfer enhancement in the microchannel.