Heat Transfer Entropy Generation Rate Research Articles

Numerical investigation on mixed convection for laminar molten salt in horizontal square tubes

Molten salts are the key working media in nuclear and new energy fields. However, the mixed convection of molten salt is rarely investigated. The mixed convection of laminar flow in horizontal square tubes with constant heat flux is analyzed using Fluent software for the first time. The effects of heat flux, inlet temperature and Reynolds number on the flow and heat transfer performances are evaluated. It is found that as the buoyancy force increases, the friction factor, flow entropy generation rate, Nusselt number, and exergy efficiency increase, while the heat transfer entropy generation rate and exergy loss decrease. Compared to the forced convection, the maximum local friction factor, flow entropy generation rate, mean Nusselt number and mean exergy efficiency for mixed convection increase by 25.8%, 50.5%, 109.0% and 7.6%, respectively. Moreover, the maximum mean heat transfer entropy generation rate and exergy loss decrease by 56.1% and 57.3%, respectively. In addition, approximately 42% of the friction factor predicted by Fluent and 30% of the heat transfer predicted by Fluent exceed 20.0% uncertainties of conventional correlations. Therefore, an accurate friction factor correlation with 12.9% uncertainty and an accurate heat transfer correlation with 9.1% uncertainty are proposed.

Numerical study on a coaxial geothermal exchanger equipped with a new inner tube: Entropy generation, thermodynamic irreversibility analysis and exergy efficiency performance evaluation

This paper focuses on the effect of inner tube fitted with CSF of different widths (H = 5–11 mm), thicknesses (W = 1.5–9 mm) and pitches (P = 50–200 mm) on the entropy generation, thermodynamic irreversibility and exergy efficiency performance of CGHE and gives the direction of improvement of CSF geometric parameters. A performance evaluation chart of exergy efficiency was also established to evaluate the exergy efficiency of each working condition. The results show that the assembly of CSF on the inner tube of the CGHE can effectively reduce the heat transfer entropy generation rate S˙gen,h, lowest 67.9% lower than that of the smooth inner tube, but the friction entropy generation rate S˙gen,f is significantly higher, highest being 10.33 times higher than that of the smooth inner tube. Increasing the width H and thickness and decreasing the pitch P both increase the entropy generation rate, but changes in width and thickness have a small effect on entropy, while changes in pitch P have a large effect on entropy, and their W˙loss follow the same pattern of variation as S˙gen. From Be and Ns it is clear that 99% is frictionally irreversible when Re > 30000, and frictional entropy generation is minimized at Re < 15000. When evaluated in terms of heat transfer improvement number NH, the direction of improvement of the CSF geometry parameters should be to increase the width H, thickness W and pitch P. In terms of exergy efficiency, the exergy efficiency for all cases results in greater than smooth inner tube coaxial GHE for the same pumping power and pressure drop conditions, but deterioration occurs for the same mass flow rate.

Local hydrothermal characteristics and temperature uniformity improvement of microchannel heat sink with non-uniformly distributed grooves

In order to improve the temperature uniformity of bottom walls, a novel design of microchannel heat sink with From-Sparse-To-Dense embowed grooves (MC-FSTD) was proposed as well as a comparative one with From-Dense-To-Sparse embowed grooves (MC-FDTS). The local hydrothermal characteristics and temperature distribution variance of bottom wall of microchannel heat sinks were numerically investigated for Reynolds number (Re) ranging from 100 to 800. The results showed that the denser grooves led to lower flow velocity, lower local friction factor and higher local pressure as well as the lower bottom wall temperature (Tw) and higher local Nusselt number (Nuz). The local Nusselt number and the bottom wall temperature of MC-FSTD were 2.17 times higher and 9.1K lower than that of MC-SC at z = 10 mm, respectively. MC-FSTD also had the most uniform temperature of the bottom wall and the corresponding temperature variance was 6.42 times lower than that of MC-SC at Re = 795.38. The heat transfer entropy generation rate and the total entropy generation rate of MC-FSTD were also the lowest with an acceptable friction entropy generation rate. Finally, the effects of the decreasing magnitude (Li) of the distance between two adjacent grooves on the local hydrothermal characteristics and temperature uniformity of MC-FSTD were explored and the total entropy generation rate was further reduced to 5.22 × 10−4 W/K at Li = 0.015 mm and Re = 795.38.

Entropy generation analysis of heat and water recovery from flue gas by transport membrane condenser

Abstract The transport membrane condenser (TMC) has been used for heat and water recovery from coal-fired power plant flue gas. The capillary condensation of water vapor in membrane pore structure is the main gas separation mode. A lumped parameter model was established to study the heat and mass transfer in TMC. Recovered water and heat flow rates, water recovery ratio, heat recovery efficiency, and pressure drops were calculated. The temperature and humidity ratio distributions were displayed. The influences of structural parameters and operating conditions on the water and heat recovery performances were analyzed. In addition, the entropy generation model was proposed to calculate entropy variations and entropy generation components. The aim is to provide insights into TMC parameter selection and operation optimization. Moreover, the relationship between entropy generation components and TMC performances were confirmed. The results show that increasing packing fraction, or decreasing the membrane inner diameter or membrane pore size can improve the heat and water recovery performances. Besides, high water flow rates and low water temperatures have advantages in the operation. Increasing the mass/heat transfer driving force can enhance heat transfer performance, but the heat transfer entropy generation rate also increases. The maximum mass transfer entropy generation rate often corresponds to the best water recovery performance.

A criterion beyond conservation equations for complex transport process modeling – A case of Rayleigh-Bénard convection

Heat or mass transfer becomes usually complicated if coupled with fluid flow. To model the process, the mass, heat, and momentum conservation equations have been well established, which are shown in our work not the only governing laws. We analyzed heat transfer entropy generation rate and viscous dissipation of coupled heat and momentum transport phenomena in Rayleigh-Bénard convection, and proposed a hypothesis that the velocity and temperature distributions can be governed by the balance between the maximization of entropy generation and the minimization of viscous dissipation. Such a criterion, combined with a multi-scale model, was used to formulate a variational problem, which was then solved to reconstruct the velocity, temperature, and Nusselt number distributions in the Rayleigh-Bénard convection. As a result, three parameters obtained can be used to correct the error caused by the computations with low resolution of meshes. We show that the proposed method consumes significantly less computational effort than a traditional DNS, and can hopefully mitigate the limit by computational burden of the traditional conservation equation based simulations.

Numerical investigation on heat transfer performance and flow characteristics in circular tubes with dimpled twisted tapes using Al2O3-water nanofluid

The present study numerically examines heat transfer and flow in circular tubes fitted with dimpled twisted tape inserts, and Al2O3-water nanofluid is employed. Considering the effects of dimples, protrusions, nanofluid volume fraction α and nanoparticle diameter dp, the discussion and analysis on heat transfer, flow characteristics, turbulence kinetic energy TKE, thermal property, entropy generation and maximum local wall temperature Tmax are detailedly provided. The results show that dimple side and protrusion side both realize great heat transfer enhancement, and dimple side behaves better compared with protrusion side. Utilization of dimples leads to an increase by 25.53% in convective heat transfer coefficient h at most compared with smooth tape. Heat transfer performance is greatly improved on both tape wall and tube wall owing to disturbance to flow structures. The overall TKE level significantly rises especially close to the core flow region when dimples are adopted, and overall swirl flow intensity greatly increase, further resulting in enhancement of turbulent mixing and heat transfer. Remarkable decline in average heat transfer entropy generation rate Sah is identified, accompanied by slight rise in average friction entropy generation rate Saf, where average total entropy generation rate Sa inclines by 29.10% at most in comparison with smooth tape. Besides, the employment of nanofluids results in great improvement of heat transfer, with further enhanced effect when α increases, accompanied by slight growth in resistance. A maximum increase by 58.96% in h is identified compared with basefluid case, and it is quite helpful that in this case the maximum rise in friction factor is only 5.05%. The wall temperature distribution is greatly improved using nanofluid, and nanofluid case provides markedly improved thermal conductivity distribution in spite of slight growth in dynamic viscosity. The heat transfer in recirculation-caused low heat transfer regions is further promoted through nanofluids when dimple technique is adopted. Utilization of nanofluid brings about a significant reduction in Sah, the effect of which is further promoted by a rising α, with a slight increase in Saf, where Sa is reduced by 28.89% at most. The Tmax greatly declines using nanofluids with further enhanced effect when α ascends. Furthermore, utilization of smaller dp gives rise to significant heat transfer enhancement compared with the larger one, with extremely mild resistance increase. Lower Sa and lower Tmax are both realized when smaller dp is used, and the effect of dp reaches an extremely low level when it exceeds 40nm.

Numerical investigation on heat transfer performance and flow characteristics in a rectangular air cooling channel (AR = 2) with ridged dimples

As an innovative type of structure, ridged dimples exhibit great potential for heat transfer enhancement. In this study, the effects of ridged dimples on heat transfer, flow characteristics and entropy generation in a rectangular air cooling channel with aspect ratio of 2:1 are numerically investigated. A comparative and comprehensive analysis is accomplished, considering the influences of extending ratio r and Reynolds number Re, as well as a comparison with the results of spherical dimple case. It is shown that heat transfer is further enhanced when ridged dimples are employed compared with spherical dimple case, and the effect is markedly promoted when r increases. The spherical dimple primarily affects the dimpled wall, while the influence of ridged dimple on heat transfer enhancement is mainly reflected on the side walls and top wall. In addition, the areas of the high temperature regions at the side boundaries of all walls significantly decrease with an increasing r. Besides, the overall turbulent kinetic energy TKE in the domain of spherical dimple case is in a relatively low level. When ridged dimples are adopted, a considerably large area of high TKE is detected in the domain above the dimpled wall. And the proportion of the high TKE region in the domain rises with the increase in r. In ridged dimple cases, the low TKE regions found in spherical dimple case at the side boundaries are not detected. The overall TKE level in ridged dimple case is higher than that of spherical dimple case. Furthermore, the average heat transfer entropy generation rate and average total entropy generation rate are prominently reduced with the employment of ridged dimples, while a slight increase in average friction entropy generation rate is detected. The employment of ridged dimple significantly reduces the maximum wall temperature and distinctly improves the wall temperature uniformity, with a further enhanced effect when r ascends, greatly beneficial for safe and stable operations of engineering devices.

Entropy Generation and Heat Transfer Performances of Al2O3-Water Nanofluid Transitional Flow in Rectangular Channels with Dimples and Protrusions

Nanofluid has great potentials in heat transfer enhancement and entropy generation decrease as an effective cooling medium. Effects of Al2O3-water nanofluid flow on entropy generation and heat transfer performance in a rectangular conventional channel are numerically investigated in this study. Four different volume fractions are considered and the boundary condition with a constant heat flux is adopted. The flow Reynolds number covers laminar flow, transitional flow and turbulent flow. The influences of the flow regime and nanofluid volume fraction are examined. Furthermore, dimples and protrusions are employed, and the impacts on heat transfer characteristic and entropy generation are acquired. It is found that the average heat transfer entropy generation rate descends and the average friction entropy generation rate rises with an increasing nanofluid volume fraction. The effect of nanofluid on average heat transfer entropy generation rate declines when Reynolds number ascends, which is inverse for average friction entropy generation rate. The average wall temperature and temperature uniformity both drop accompanied with increasing pumping power with the growth in nanofluid volume fraction. The employment of dimples and protrusions significantly decreases the average entropy generation rate and improve the heat transfer performance. The effect of dimple-case shows great difference with that of protrusion-case.

Thermodynamic analysis of lead–bismuth eutectic turbulent flow in a straight tube

The entropy generation in Lead–Bismuth Eutectic (LBE) turbulent flow in a cooled straight tube with a diameter of 10 mm and a length of 1 m is numerically investigated taking into account the temperature-dependent of LBE properties. The standard k-epsilon turbulence model combined with appropriate turbulence Prandtl number is adopted to describe the turbulent heat transfer and flow of LBE under different heat flux and flow conditions. The average cooling heat flux of 5 × 105 W/m2, the flow inlet temperature of 723.15 K and the flow velocity of 1–3.5 m/s are used in these studies. The heat transfer entropy generation rate is far larger than frictional entropy generation rate, and the heat transfer entropy generation rate distributes more widely than frictional entropy generation over the flow cross-section. Under the same heat transfer area and heat load conditions, the thermodynamic performance of the LBE flow under considered conditions is the best under a slight increase of the absolute heat flux along flow direction condition, followed by the constant heat flux condition; the decreasing absolute heat flux along flow direction is the worst.