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• The liquid metal cross flow tube bundle under pulsation condition is simulated. • The effect of different pulsation frequency and amplitude are analyzed. • Circumferential flow field and temperature field of the single tube are analyzed. • The vortex characteristics and turbulent structure are analyzed with PSD and CWT. Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications. [ABSTRACT FROM AUTHOR]

• A reservoir-assisted loop thermosyphon with a micro-grooved evaporator (MGTPLT) was fabricated. • Micro-grooved structures allowed faster startup and more stable operation of the MGTPLT. • The convection heat transfer ratio was proposed to evaluate the heat transfer enhancement. • The MGTPLT reduced evaporator temperatures and achieved thermal performance enhancement. The two-phase loop thermosyphon (TPLT) offers significant advantages in high-efficiency and long-distance heat transport for various thermal and energy-related applications without external power consumption. In this paper, the startup and heat transfer characteristics of a reservoir-assisted loop thermosyphon with a micro-grooved evaporator (MGTPLT) were experimentally investigated and compared with a loop thermosyphon having a smooth-tube evaporator (STPLT). R245fa was used as a working fluid at volumetric filling ratios of 50–80 %. The results demonstrated that the additional microgrooves allowed a shorter startup time characterized by a lower maximum temperature with respect to the temperature overshoot. Compared to the STPLT, the MGTPLT exhibited lower evaporator temperatures and better temperature uniformities; the micro-grooved structures could alleviate local intermittent dry-out at moderate and high levels of power input for filling ratios of 50 and 60 %. The addition of micro-grooved structures enhanced the heat transfer performance of the TPLT characterized by lower thermal resistances. The convection heat transfer ratio (CHTR) was proposed to evaluate the enhancement of the convection heat transfer within the evaporator region after using microgrooves. The CHTR showed a decreasing tend with increasing the power input. At moderate filling ratios, the value of the CHRT was greater than 1.1, and an approximately 18 % increase in the convection heat transfer was achieved under the combined condition of 60 % filling ratio and 250 W power input. The fundamental mechanism of heat transfer performance enhancement with the aid of micro-grooved structures was analyzed and elucidated. [ABSTRACT FROM AUTHOR]

· Different change rate of cross-sections along the flow field is investigated. · Microchannel condenser is used in loop heat pipe. · Effects of operating height situation and microchannel structure are studied. · Smooth operation and anti-gravity performance are effectively improved with the help of tapered channel. The microchannel condenser has the advantages of high condensation efficiency and low flow resistance. The vapor can liquefy and return to the evaporator rapidly, and the heat transfer performance and the smooth operation of the loop heat pipe can be improved. Suitable change rate of the cross-section along the microchannel flow field can adjust the pressure drop and flow rate of working fluid, and improve the anti-gravity performance of the loop heat pipe. In this paper, microchannels with different change rates of cross-section along the flow field, including 0.5 mm straight channel, 1 mm straight channel, divergent channel, and tapered channel, are designed and applied as condensers in loop heat pipes. The entrance size of the gradient microchannel is 0.5 mm or 1 mm, which uniformly expands to 1 mm or shrinks to 0.5 mm along the flow field. The vapor and cooling water form a countercurrent direction of motion in the microchannel, and the heat exchange is carried out through the bottom of the channel. The influence of the microchannel condenser structure and operating height situation between the evaporator and condenser on the heat transfer performance of the loop heat pipe are experimentally investigated. The results show that the loop heat pipe achieves an extremely low thermal resistance of 0.004℃/W with gravity assist. Under the gravity-free operation situation, the maximum evaporator temperature of the loop heat pipe with 0.5 mm straight channel and 1000 W heat load is 128.1 °C, and the vapor pressure drop fluctuated is in the range of -1000 ∼ 2500 Pa. Under the anti-gravity operation, the loop heat pipe with the tapered channel has effective transfer performance at 500 W, and the influence factor is only 28.1 %. [ABSTRACT FROM AUTHOR]

• Presents an analytical solution for thermal spreading in a cylindrical body with spatially varying side wall cooling. • A series solution is derived, with coefficients shown to be the solution of a set of linear algebraic equations. • Impact of key non-dimensional parameters on temperature field and thermal resistance is investigated. • Results are shown to be in excellent agreement with past work for special cases. • Results are of relevance for edge cooling of microprocessor chips as well as batteries. Thermal spreading and constriction resistance play an important role in thermal management of a variety of engineering systems, including semiconductor devices. Moreover, thermal contact resistance at interfaces between materials is also governed by spreading and constriction processes. Most of the past literature on theoretical modeling of thermal spreading from a source into a larger body assumes an adiabatic side wall, whereas several recent thermal management problems involve active cooling at the side wall. This work addresses such scenarios by solving the thermal spreading problem in a finite thickness cylinder with spatially varying convective heat transfer on the side wall. An eigenfunction-based series solution for the temperature field is written, the coefficients of which are shown to be governed by a set of coupled algebraic equations. Results are shown to agree well with past work for special cases of the general analysis presented here. Thermal spreading resistance is computed for a number of practical scenarios, including step function, periodic and sinusoidal convective cooling on the side wall. In each case, it is shown that the temperature distribution is consistent with the nature of the spatially varying boundary condition. A key theoretical insight from this work is that under certain conditions, the classical definition of thermal spreading resistance may become negative due to the presence of two heat sinks in the problem. Therefore, analysis based on total resistance may be more appropriate. This work offers a significant generalization of past papers, making it possible to analyze realistic thermal management problems, in which, side wall cooling plays an important role. [ABSTRACT FROM AUTHOR]

The influence of thermal interface resistance is an important design problem for the optimisation of heat transfer systems. For thermoelectric generators (TEG), it can limit the ability to convert thermal energy directly to electrical energy. This paper describes an experimental and numerical study of the influence of thermal interface resistance on TEG module performance. A test apparatus was constructed to evaluate TEGs under varying temperature and heat source/sink conditions. The effect of heat source/sink interface finish grades, corresponding to medium quality (N8-N6) and very fine (N5-N4) surface finishes, at varying clamp forces, is evaluated. Then the application of cold spray additive manufactured copper, as a novel method of TEG module thermal interfacing is explored. Experimental data is accompanied with numerical analysis using TEG ideal equations and the effective material properties approach with the incorporation of temperature dependency and fabrication quality factors to improve modelling accuracy. A system of simultaneous heat flow equations is then established to numerically solve for the presence of thermal interface resistance. The results show that high clamp forces (3600 N) and very fine finish grades were sufficient to minimise thermal interface resistance to the range 0–5.98 × 10 − 5 m 2 K W − 1 , representing a loss of 0–9.9% in maximal achievable power. For the cold spray thermally interfaced TEG module, resistance estimates were lower and in the range 0–5.4 × 10 − 6 m 2 K W − 1 , corresponding to a 0–1.0% loss in power. • Heat source/sink surfaces, with varying finishes (N8-N4) and clamp forces, are evaluated for influence on thermoelectric generator (TEGs) performance. • Cold spray additive manufacturing is proposed and experimentally evaluated as a novel method of TEG heat source/sink thermal interfacing. • A system of heat flow equations are used to solve for the presence of thermal interface resistance (R T I R) at TEG heat source/sink interfaces. • High clamp forces (3600 N) and very fine finishes (N4) minimised R T I R to the range 0–5.98 × 10 − 5 m 2 K W − 1 , representing a loss of 0–9.9% in maximal achievable TEG power. • R T I R estimates for the cold spray thermally interfaced TEGs were lower and in the range 0–5.4 × 10 − 6 m 2 K W − 1 , corresponding to a 0–1% loss in power. [ABSTRACT FROM AUTHOR]