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The quantification of the flow irreversibilities is crucial to developing future turbomachinery. Traditionally, design correlations and efficiencies are based on comparisons to isentropic relations to quantify the deviation from an "ideal" case. However, isentropic relations typically assume one-dimensional adiabatic flow, representing a significant departure from the actual situation in turbines operating with large levels of heat transfer. In this paper, the aerothermal losses and power generation in reversible processes are derived for three-dimensional compressible flow considering heat transfer effects. A new definition of the power potential in a reversible process is proposed that can be used to perform detailed budgeting of the losses. The proposed new loss framework can be adopted locally, which enables the precise identification of the loss source. New aerodynamic and aerothermal efficiency expressions are proposed to assess shaft power extraction in combination with heat exchange. The new method enables designers to identify the regions with high loss generation in steady and also in unsteady flow conditions, offering new avenues to evaluate the loss generation mechanisms during the design phase. Therefore, our new tool is essential to developing reduced-order models for the design of future fluid machinery. • Pioneering method for time-resolved loss production in compressible fluid machinery. • A novel approach based on volume integral compliant with heat flux. • First demonstration in transonic turbines with endwall heat flux. • Implementation suitable for experimental control volume assessment. • Proposed new correlations on thermal loss generation. [ABSTRACT FROM AUTHOR]

• Hydrodynamic instability will induce natural frequencies in steady flow and Kelvin-Helmholtz instability appears only at low Reynolds numbers. • A periodic process of vortex generation-shedding-fusion is found behind each block in pulsating flow. • As the forcing frequency St F of the pulsating flow equals the natural frequency St N in steady flow, pulsating flow improves heat transfer, especially at low Reynolds numbers. This paper presents a numerical investigation on flow and heat transfer in a grooved channel formed by 9 rectangular blocks with steady and pulsating flow. Large eddy simulation is used to simulate the hydrodynamic instability in steady flow and heat transfer enhancement for pulsating flow. The simulations for steady flow at Re =700, 1500, 2500, are firstly conducted, showing that there are 23Hz, 70Hz and 130Hz natural frequencies f N induced by hydrodynamic instability (corresponding to Strouhal number St N =0.876, 1.243, 1.385) behind the first block, which disappear quickly with the flow development. A natural frequency f N of 34Hz (St N =1.295) is found only at Re =700 and it can be maintained and amplified by groove structures. Subsequently, a sinusoidal pulsating flow is applied, with the forcing frequency St F equal to the natural frequency St N in steady flow and the dimensionless amplitude A fixed at 0.2. Flow and thermal performance of the grooved channel is studied under the pulsating condition. A periodic process of vortex generation-shedding-fusion is found behind each block and the fluid in the channel has higher turbulent kinetic energy than that under the steady condition. Ultimately, compared with the thermal and pressure-drop performance between steady and pulsating flow, the enhancement of time-average Nusselt number ranges from 48.08%, 15.64%, 8.61% and time-average friction factor increases by about 40% at Re =700, 1500, 2500. The thermal performance factor is increased by 30.96% and 4.20% at Re =700, 1500 but is reduced slightly by 3.48% at Re =2500. It indicates that the augmentation of heat transfer through pulsating flow is significant at low Reynolds numbers. [ABSTRACT FROM AUTHOR]

• The emergence of deterioration and oscillation relates with the change of flow state. • Oscillation is driven by transition flow between partial and full re-laminarization. • The estimation criteria of HTD and oscillation are given with shearing-stress model. Trans-critical CO 2 Rankine cycle has great potential in converting low-grade heat into electricity, because of its good temperature glide matching between working medium and heat source. However, the drastic variation in thermo-physical properties of CO 2 around the pseudo-critical region causes the abnormal flow and heat transfer behaviors. In this paper, the heat transfer deterioration and thermal oscillation in the heat addition of trans-critical CO 2 cycle are investigated by experiments. The test section is heated uniformly by a DC power (0∼6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for the all cases. A shear stress reconstruction model is corrected by introducing the velocity profile of turbulence in the boundary layer to attempt to provide a quantitative criterion for deterioration and oscillation. The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation is elucidated by connecting the shearing-stress reconstruction model and experimental results. It is found that, the thermal acceleration in the near-wall zone provides the key motivation to drive the heat transfer deterioration. The thermal oscillation is driven by the transition between partial and full re-laminarization flows in the local zone of heat transfer deterioration. When ( Δ τ a / τ w) < 1 & ( Δ τ / τ w) t o t > 1 or 0.1 < ( Δ τ / τ w) t o t < 1 , the turbulence is partially re-laminarized. When ( Δ τ a / τ w) > 1 & ( Δ τ a / τ w) > > ( Δ τ b o / τ w) , the intense thermal acceleration causes the turbulence to be fully re-laminarized in the near-wall zone and it maintains the new laminar boundary. The turbulence is partially re-laminarized in the lower boundary and it is almost full re-laminarization in the upper boundary of the oscillation wall temperatures. The results confirm that the emergence of heat transfer deterioration and oscillation instability in mixed turbulent convective relate with the change of flow states caused by acceleration and buoyancy effects. [ABSTRACT FROM AUTHOR]

• Thermal management of far-infrared barrel heating in injection molding is studied. • Excess heat is real-time predicted and reused during the heating process. • A comprehensive heat transfer model of excess heat generation is developed. • A framework combing predictive control and excess heat prediction is proposed. • Energy efficiency and temperature accuracy are achieved at the same time. Injection molding accounts for a huge share of the energy consumption of material processing. It has received considerable attention for its potential to enhance production efficiency, which relies on both product consistency and energy efficiency. Far-infrared heating, which has the advantages of high energy efficiency and low dissipation, has emerged as a promising heating technology in injection molding. However, the issue of low heat dissipation can lead to excess heat accumulation and cause substantial temperature errors. In this paper, a new barrel heating control method based on generalized predictive control (GPC) with real-time excess heat prediction model is proposed, in order to improve control accuracy and heating efficiency of far-infrared heating. A constrained multistep optimization combined with excess heat compensation is stated and an experimental platform is designed to validate the superiority of the proposed method under different operation scenarios. Experimental results show that the proposed method outperforms traditional proportional-integral-differential (PID) and GPC methods. Specifically, the proposed method can improve temperature control accuracy by 85.3 and 78.9 %, compared to PID and GPC, respectively. Such high temperature accuracy goes a long way in improving product consistency. Moreover, energy efficiency is improved, e.g., heating time is reduced by 42.7 and 16.5 %, energy consumption is reduced by 27.9 and 25.4 %, compared to PID and GPC, respectively. Control robustness is also verified under different operating conditions. In second-heating experiments, the temperature accuracy is improved by 84.4 % compared to PID. Therefore, the proposed method has the potential to enhance the production efficiency of injection molding. [ABSTRACT FROM AUTHOR]

• Semi-analytic methods are used to solve 3D DPL problems. • Compared with FEM, the semi-analytic method has higher computational efficiency. • The phase lag of heat flow and the phase lag of temperature gradient have influence on the temperature response speed and the formation of temperature field. With the development of picosecond laser technology, the dual-phase-lagging (DPL) model is more suitable to describe the heat transfer problem with micro-scale effect and micro-time effect. In this paper, a three-dimensional dual-phase-lagging heat transfer model for picosecond laser processing is established. Then Fourier transform (FT) and Laplace transform (LT) are used to derive the frequency response functions (FRFs) of the problem in the frequency domain, and the inverse Laplace transform (ILT) and discrete-convolution fast Fourier transform (DC-FFT) algorithm are used to solve the temperature field. The calculation accuracy of this method is verified and the effect of phase lag constants on temperature field is studied. The results show that the semi-analytical method has a high accuracy for solving the dual-phase-lagging heat conduction model, and the phase lag of heat flux will affect the heating rate in the heating stage, and the phase lag of temperature gradient will delay the formation of temperature field. The effect of phase lag constants on temperature-time history is investigated and the related phenomena are explained by using the two-step heat transfer theory. This semi-analytical method provides a new approach to solve the problem of three-dimensional dual-phase-lagging heat transfer. [ABSTRACT FROM AUTHOR]

• Residual flexural capacity tests of reinforced alkali-activated slag-based concrete beams after exposure to fire. • Comparisons between flexural behaviour of RC beams using alkali-activated slag-based concrete and OPC concrete after fire. • A simplified analytical model was adopted to predict the residual flexural capacity of RC beams. Understanding the flexural behaviour of reinforced alkali-activated slag-based concrete beams after exposure to a fire is imperative to ensure their safety use and develop adequate retrofitting measures. However, up to this point in time, no relevant investigations on this specific subject have been reported. This paper presents an experimental study comparing the flexural behaviour of alkali-activated slag-based concrete (AAC) and Portland cement concrete (PCC) beams exposed to the standard ISO 834 fire curve. The research considers concrete strength and heating duration as research parameters. The test results revealed that AAC beams displayed a lower temperature field and experienced less mid-span deflection increase compared to PCC beams during heating. Normal-strength AAC and PCC beams exhibited comparable losses in flexural capacity after being heated for 1 h or 1.5 h. However, PCC beam with a concrete strength over 80 MPa experienced more capacity loss due to severe spalling during heating. On the other hand, compared with PCC counterparts, AAC beams exhibited lower residual flexural stiffness and experienced greater stiffness loss, mainly due to the formation of more cracks induced by thermal shrinkage of AAC. The flexural capacity and stiffness loss of AAC beams increased with increasing concrete strength and heating duration after fire exposure. The residual flexural capacities of RC beams were predicted using a simplified analytical model that incorporates heat transfer analysis and the 500 ℃ isotherm method. The predictions demonstrated excellent agreement with the test results. [ABSTRACT FROM AUTHOR]

The double-wall structure has attracted wide attention, especially in the cooling design of the turbine vane, because of its excellent cooling performance. However, extreme flow resistance becomes the main obstacle limiting the application of double-wall structure. Previous studies focus on further improving the cooling performance of the impingement/effusion cooling system, and few improved schemes have been made to reduce the flow resistance. In this paper, a novel cooling structure with hollow pin fins is proposed, which takes into account both the improvement of the cooling effect and the reduction of flow resistance. The three-dimensional steady-state numerical simulation obtains the film cooling effectiveness, Nusselt number, overall cooling effectiveness, and coolant flow coefficient of the novel and traditional double-wall structures. The gird uses tetrahedral elements, and the turbulence model selects the shear stress transport (SST) k-ω model for numerical calculation. The effects of different hollow pin fin arrangements on the cooling performance and flow resistance characteristics are investigated to study the novel cooling structure. The main conclusions are as follows: In terms of external cooling, placing pin fin holes near the film holes can reduce the vortex intensity and improve the film coverage. The surface-averaged film cooling effectiveness is increased by 112.5% compared to the traditional cooling structure. In internal cooling, the novel cooling structure has a more considerable spanwise distance between the impingement holes and a lower impinging jet velocity, reducing the target plate's heat transfer. However, the novel cooling structure still has excellent internal cooling owing to the increase of heat transfer area by the pin fin. The novel cooling structure maintains internal cooling and significantly improves external cooling, increasing the surface-averaged overall cooling effectiveness by up to 14.5%. In terms of coolant flow resistance, adding the hollow pin fin provides a lower total pressure loss option for coolant outflow. The novel cooling structure brings the maximum increase of flow coefficient of 32.4% and reduces the flow resistance significantly. • A novel double-wall with hollow pin fins is proposed. • The double-wall with different arrangements of the hollow pin fins is investigated. • The novel double-wall increase the film cooling effectiveness by up to 112.5%. • The novel double-wall maintains the excellence of impingement cooling. • The novel double-wall increase the overall cooling effectiveness by 14.5%. • The novel double-wall brings an increased flow coefficient of 32.4%. [ABSTRACT FROM AUTHOR]

To improve the heat dissipation efficiency of the microchannel, this paper designs a microchannel with five types of column inserts with different spiral distributions and numerically investigates the heat transfer and flow characteristics under different geometric structures. The five types of column inserts with different spiral distributions in the order of no insert, clockwise, counterclockwise, same direction double helix structure, and full distribution. All five model structures have the same flow path area, which can more visually demonstrate the effect of spiral bar inserts on heat transfer. The simulation results show that the heat sink with column inserts of spiral distribution has better heat transfer performance than the heat sink without column inserts, due to the column inserts in the flow channel enhance the flow mixing and achieve better heat transfer. The different distribution of the columnar inserts also affects the number of columnar inserts, and the pressure drop decreases as the number of columnar inserts decreases. Among all the considered, the counterclockwise spiral distribution has the best heat transfer performance, which is 5–10% higher than that of the structure without inserts. This is since the column inserts of counterclockwise spiral distribution is more conducive to fluid heat transfer, providing a theoretical basis for improving radiator performance. [ABSTRACT FROM AUTHOR]

Deep reinforcement learning (DRL) has gradually emerged as a novel and effective method for intelligent control of conjugate heat transfer. Through proper training, DRL agent usually can find a better control strategy than the one optimized manually. For numerous numerical simulation-based investigations, the promising results have only been obtained on 2-dimensional models due to the heavy burden of data acquisition. This paper proposes a novel strategy of transferring the control policy learned in 2-dimensional environment into a new 3-dimensional environment. PDD-DQN (Prioritized Dueling Double Deep Q-Networks) algorithm is used to recognize the underlying relationship between the forced fluid flow and heat transfer performance to produce a control strategy. The DRL controller is first trained on a simple 2-dimensional cavity with one heat source, and the DRL controller is able to reduce the maximum temperature to 315 K which is 2 K lower than the manually optimized control strategy; then the control strategy is transferred to 3-dimensional models where the maximum temperature is further reduced to 308 K; and compared to training the DRL agent directly on 3-dimensional model, the policy-transfer strategy requires only 10% computational expenditure. The proposed strategy is then applied to a more complex testbed with multiple heat sources for further investigating its ability; in both 2D and 3D environment, the DRL controller can cool the maximum temperature to be 307 K which is 8 K lower than the manually optimized control policy; and the computational cost drop to be 3%. The generalization ability of the trained DRL agent in inter-dimensional geometries is confirmed. For the cases with huge grid size, the policy-transfer strategy can economize exponential computational cost, which is of great significance for applying DRL methods to practical thermal control problems. [ABSTRACT FROM AUTHOR]

The rapid advancement of micromachining technology has led to the proliferation of micro-combustors in both civilian and military applications, and the actual operation process requires the ability to adapt to complex working conditions. The sudden change of the operating conditions poses a huge threat to the flame stability in micro-combustion, but the flame regulation, especially the adaptive regulation, has proved to be challenging under the sudden change of inlet velocity. In this paper, we report a novel method that significantly improves combustion performance, including heat transfer enhancement under steady-state conditions and adaptive stable flame regulation under velocity sudden increase. We successfully harnessed the synergistic benefits of both the front and rear baffles, investigated the angle matching of the baffles at various inlet velocities, and employed nitinol memory metal as the baffle material to examine its impact on enhancing heat transfer and flame control behavior. According to the results, under the synergic action, the combustion efficiency reaches to 96.92%, and the baffle angle matching 60° + 30° shows good combustion performance at medium-low velocity, and a flame breaking limit of 92 m/s is obtained at 0° + 0° with high-velocity condition. In addition, nitinol alloy is selected as the deformation baffle material, the flame breaking limit is further extended by 6 m/s, and the combustion efficiency can still be maintained at 0.45 ms by a velocity sudden increment of 60 m/s, and the pressure loss is reduced by more than 70%. This study is helpful to provide a reference for the steady combustion under the sudden change of micro-scale conditions, and provides a new idea for the adaptive control of flame. [ABSTRACT FROM AUTHOR]

• Different strategies are analyzed to enhanced microchannels based heat sinks. • Nanofluids to enhance heat performance of microchannel based heat sinks is analyzed. • Experiments are performed for Al 2 O 3 , Au and Ag nanoparticles at various concentrations. • The flow-boiling regimes are identified and discussed for the working conditions considered. • Two-phase flow conditions may not always be beneficial, low concentrations of nanoparticles (ϕ ≪ 1%) can have a notable impact on heat transfer mechanisms. This paper investigates the advantages and challenges associated with two-phase flows, specifically flow boiling of pure liquids and nanofluids, for cooling applications in microchannel heat sinks. The study explores various two-phase flow patterns, their related issues, and examines the potential of nanoparticles to enhance heat transfer. Alumina (Al 2 O 3), gold (Au), and silver (Ag) nanoparticles at different concentrations were tested. Experimental tests were conducted under different working conditions using various working fluids, including water, Al 2 O 3 1 wt%, Ag 1 wt%, Au 1 wt%, Au 0.75 wt%, Au 0.5 wt%. The heat fluxes used were 1.026 kW/m2, 1.696 W/m2 and 2.403 kW/m2, while the volumetric flows ranged between 0.5 mL/min and 1.5 mL/min. The observed results indicate that even for the lowest particle concentration tested, the water-Au nanofluid exhibits superior cooling performance compared to the other examined fluids. The findings suggest that although two-phase flow conditions may not yield significant benefits, even small concentrations of nanoparticles (ϕ ≪ 1%) can significantly impact heat transfer mechanisms. This approach provides a cost-effective and efficient alternative for cooling microchannel heat sinks without necessitating the use of two-phase flow conditions. [ABSTRACT FROM AUTHOR]