• Energy Research

As the continuing integration and size deflation of component dimensions in electronic circuits and increase in the number of transistors in modern microprocessor chips, especially for heat dissipation of micro/nano scale devise, traditionally used single phase fluid cannot meet the requirements for highly efficient heat transfer, which thus frequently results in the damage of electrical devices. Consequently, thermal conductivity enhancement of working fluids is of great significance for advanced thermal energy conservation and conversion. Nanofluids, which possess a superior thermal conductive performance, are studied towards an alternative to the traditionally used working fluids, have attracted ample attention within the past decades. In this paper, firstly, we summarized the recent progress in the preparation of nanofluids, in particular for a method involving a covalent concerning reorganization or generation; subsequently, the utilization of nanofluids in hitherto unsummerized micro/nano scale heat and mass transfer fields, especially for some chemistry relating applications were discussed. All works demonstrated in this review are aiming at clarifying the fact that advanced material technologies are required in preparation of recent nanofluids on the premise of continuing harsh energy transfer situation; on the other hand, nanofluids were also able to offer insights for novel micro/nano scale energy transportation which has not yet been reviewed before.

Abstract Based on a retrofitted optical engine, high-speed photography technology is applied to obtain combustion flame patterns within the cylinder. The flame temperature and the space-time distribution of the characterize soot concentration (KL factor) are calculated using two-color method. Effects of coal-based synthetic fuel (CTL) and its mixture with oxygenated fuels on the combustion and soot generation process of the engine are experimentally studied. Two additional fuels are used in this study; one is alcohol-based fuel, the n-butanol and the other one is dimethyl carbonate (DMC). Result shows that when oxygenated fuels are mixed into the CTL, the ignition timing is postponed, and the ignition delay is prolonged. At the same time, with a shortened ignition duration caused by the additional oxygenated fuels, the flame area and flame brightness are significantly reduced. For the CTL and DMC blends, as the percent of DMC increases, both the combustion temperature within the cylinder and the KL factor are observed reducing in gradient, those are favorable for inhibiting formation of the soot. Comparison between the two mixed fuels shows that, with a same oxygen fraction, the temperature distribution of the CLT/n-butanol blends presents a bimodal form, and the average in-cylinder temperature is lower than that of the CTL/DMC blends, these characters are conductive to low-temperature combustion. With regards to the KL factor, the CTL/n-butanol mixture has a stronger effect on reducing the KL factor compared to that of the CTL/DMC blends.

Stable TiO2-H2O nanofluids are prepared and their stabilities are studied. An experimental set for studying the heat transfer and flow characteristics of nanofluids is established. Heat transfer and flow characteristics of TiO2-H2O nanofluids in a circular tube with rotating and static built-in twisted tapes are experimentally investigated and compared. An innovative performance evaluation plot of exergy efficiency is developed and the exergy efficiency of tube with rotating and static built-in twisted tapes filled with nanofluids is analyzed in this paper. The results indicate that the combination of rotating built-in twisted tape and TiO2-H2O nanofluids shows an excellent enhancement in heat transfer, which can increase the heat transfer by 101.6% compared with that of in a circular tube. The effects of nanoparticle mass fractions (ω= 0.1%, 0.3% and 0.5%) and Reynolds numbers (Re = 600–7000) on the heat transfer and flow characteristics of TiO2-H2O nanofluids are discussed. It is found that there is a critical Reynolds number (Re = 4500) for the maximum value of relative heat transfer enhancement ratio. The comprehensive performance of the experimental system is analyzed. It can be found that the comprehensive performance index of the experimental system firstly increases and then reduces with Reynolds number, and it can reach 1.519 at best. However, for the performance evaluation of exergy efficiency, the coupling of rotating twisted tape and nanofluids deteriorates the exergy efficiency. Also, it can be found that the exergy efficiency of the circular tube with twisted tape is greater than that of circular tube under the same pumping power and pressure drop, but it shows deterioration under the same mass flow rate.

In this work, a three-dimensional transient numerical model of a thermoelectric generator module considering the temperature-dependent properties and the topological connection of load resistance is proposed to study its dynamic response characteristics. The dynamic output power and conversion efficiency of the thermoelectric generator module under steady and different transient temperature excitations are compared and studied. A time delay exists in the output response of the thermoelectric generator module, and the time delay increases when the temperature rate increases. When the heat source temperature changes rapidly, the corresponding output power, conversion efficiency, and other thermal responses will show a more stable change trend. Moreover, the dynamic response characteristic of the output power is synchronous with that of the conversion efficiency. The periodic temperature excitation may amplify the output power, where the average output power of the sine and triangle waves are 4.93% and 2.82% respectively higher than the steady-state output power. However, the average conversion efficiency of both is almost identical to the steady-state conversion efficiency. The proposed model contributes to predicting the dynamic performance of thermoelectric generators, and can be further extended to the whole thermoelectric generator system.

A combined experimental and numerical study has been conducted to compare the characteristics of free jet flames and confined jet flames from a small ceramic tube burner using liquid ethanol as fuel. The visualization results show that the flame shapes between the free jet flame and the confined jet flame have some differences. An obvious liquid–vapor interface has been found for the confined jet. Numerical results of flame lengths and flame temperature show reasonable agreement with the experimental data within the present parameter ranges. At the same liquid ethanol flow rate, the flame length of confine jet is smaller than that of free jet. For the confined jet, the heat loss is smaller; the flame temperature is higher, so the enhanced diffusion results in a shorter length.

The present study explores the parametric investigation of a heat sink filled with composite of pure phase change material (PCM), nanocomposite phase change material (NCPCM), metal-foam (MF) by employing the numerical approach for effective passive thermal management of electronics. The combinations of heat sink are varied by filling PCM, NCPCM, MF+PCM and NCPCM+MF. Different parameters such as MF materials, porosities, pore densities (PPI-pores per inch), volume fractions of nanoparticles in NCPCM, power levels and combination of MF+NCPCM by varying different porosities and nanoparticles volume fractions. Copper (Cu) nanoparticles of 1%, 3% and 5% volume fraction were dispersed in RT-35HC, used as a PCM, and copper, aluminium (Al) and nickel (Ni) MFs were embedded inside the heat sink. Transient simulations with conjugate heat transfer and melting/solidification schemes were formulated using finite-volume-method (FVM). The thermal performance and melting process of the NCPCM filled heat sink were evaluated through melting time, heat storage capacity, heat storage density, rate of heat transfer and rate of heat transfer density. The results showed that with the addition of Cu nanoparticles and MF, the rate of heat transfer was increased and melting time was reduced. The melting time was reduced by 1.25%, 1.87% and 2.34%; and rate of heat storage is enhanced by 1.35%, 0.76%, and 0.19% with the addition of 1%, 3% and 5% volume fraction of Cu nanoparticles, respectively. The composite of MF+NCPCM showed the lower heat sink temperature and higher liquid-fraction were obtained. The latent-heating phase duration was decreased with the increase of Cu nanoparticles volume fraction. Additionally, the lower reduction in melting time of 18.10% and higher rate of heat transfer of 8.12% were obtained with 1% Cu nanoparticles, 95% porosity and 10 PPI Cu MF based heat sink.

This two-dimensional numerical study explores the thermal performance of a hybrid nanocomposite phase change material (HNCPCM) dispersed metal-foam with open slots embedded heat sink for passive cooling enhancement of electronics. The hybrid nanoparticles of GO-Ag of varying volume fractions from 0% to 6% are dispersed into the RT-28HC, used as a phase change material (PCM). The metal-foam strips of constant width and height are embedded inside the heat sink to improve the heat transfer rate and melting process of PCM at a constant power level. Unsteady simulations are conducted to solve the governing equations numerically using the Finite Volume Method (FVM) scheme. The results depicted that employing HNCPCM has better heat transfer enhancement compared to the pure PCM because of the addition of nanoparticles. The 2% volume fraction of GO-Ag showed the higher melting time and optimum heat transfer rate. The composite of HNCPCM/metal-foam strips showed better uniformity in the melting of PCM and lower heat sink temperature is achieved which results in better passive cooling performance for electronic devices.