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• A novel performance-enhancement strategy for SLGHP geothermal system is proposed. • It relies on arousing inter-layer groundwater crossflow in reservoirs of multi-aquifers. • A numerical model is developed to describe and study the enhanced SLGHP system. • Effects of related parameters are comprehensively studied. The super-long gravity heat pipe (SLGHP) is a novel down-hole heat exchanger (DHE), which is in fast-developing and extremely suitable for deep-earth geothermal energy exploitation. The SLGHP itself has very high heat transfer coefficient, making the poor heat transfer capability of the surrounding geothermal formulations become the bottleneck constraining the overall performance of the SLGHP geothermal system. Inspired by the enhancing effect of the flowing groundwater in the aquifers on the thermal performance of the traditional DHE system, the present work proposes a heat transfer enhancement strategy based on arousing inter-layer crossflow in wellbore-connected multi-aquifers for the SLGHP geothermal system. A detailed numerical study is conducted to examine the effects of key parameters like the permeability and thickness of aquifers, the distance and pressure difference between aquifers. It is found that: i) a larger aquifer permeability leads to larger heat extraction rate of the SLGHP, but the heat extraction rate increment decreases due to the marginal effect when the aquifer permeability is larger than 10−12 m2; ii) a larger pressure difference improves the heat extraction of the SLGHP, the groundwater flow pattern from the deep to the shallow aquifers rather than the reversed pattern is found to be more beneficial due to the geothermal gradient; iii) the distance between aquifers shows a composite impact on the heat extraction performance of the SLGHP. A larger distance not only enlarges the heat transfer area between the SLGHP and the groundwater, but also creates an impeding effect on the heat uptake of SLGHP from the geothermal formation owing to the presentence of temperature-lowered groundwater in the flow path ending-part in the wellbore. In addition, the aquifer's thickness is found to have great impacts on the SLGHP heat extraction rate, and the "cask" effect may be encountered when the thickness difference between the connected aquifers is considerably large. [ABSTRACT FROM AUTHOR]

Thermal management of fuel cell vehicles is posing a constant challenge due to the growing installed power of the fuel cell stack and consequent heat-overloaded situations of the radiator. Based on the "time-dependent" idea that considers the fluctuated heat generation and the intermittent heat peak during the journey, a novel thermal management system that employs a heat-peak regulator is proposed in the present work. The heat-peak regulator is a thermal accumulator filled with phase-change material that separately exchanges heat with the fuel cell coolant and the air conditioner refrigerant. It temporarily receives the excess heat that cannot be released out by the radiator when the heat peak occurs; later on, when the heat peak is gone, the heat is transferred to the refrigerant to remove it from the condenser. The system simulations based on the developed thermal models demonstrate that, the thermal runaway of the fuel cell stack can be eliminated or effectively weakened by this novel thermal management system, relying on the amount of the phase-change material filling the heat-peak regulator. In the present study, the 135 s and 250 s thermal runaway durations can be shortened to 0 s and 105 s, respectively, in the standardized New European Driving Cycle and World-wide Harmonized Light Duty Test Cycle under 38 °C summer weather, and the maximum temperature for the latter can be reduced from 89 °C to 83 °C. This work can make a significant contribution towards the solution of thermal management problems of the fuel cell vehicles. • Proposing the "time-dependent" thermal management methodology. • The thermal management system contains a heat-peak regulator. • The heat-overload problem for the radiator of fuel cell stack is properly handled. • The proposed system is analyzed based on a self-developed thermal model. [ABSTRACT FROM AUTHOR]

Laminar forced convective heat transfer is studied for the purpose of getting the best heat transfer performance with the least flow resistance increase. The variation calculus method is employed to establish the equations describing the optimized fluid velocity field and temperature field. Numerical solutions of the equations for a convective heat transfer process in a section-cut of a square duct indicate the optimized flow should have a transverse secondary swirl flow pattern consisting of multiple vortexes with identical swirl direction in the junction region of any two neighboring vortexes. We then propose the convective heat transfer enhancement method relying on excitation of transverse secondary swirl flow. To validate this method, we numerically study the heat transfer and flow resistance characteristics of laminar flows in tubes with four-reverse-vortex-generator (FRVG) inserts, four-homodromous-vortex-generator (FHVG) inserts, or a twisted tape insert. The calculated transverse secondary flow in the tube with the FRVG inserts approximately follows the optimized flow pattern and the tube is thus found to have the best thermo-hydraulic performance, validating the proposed convective heat transfer enhancement method. [ABSTRACT FROM AUTHOR]

Abstract: Effects of reducing the upwind area of conical-strip tube inserts on heat transfer and friction factor characteristics of turbulent flow are experimentally investigated. The conical-strip insert resembles one quarter of the conical-ring insert in terms of its upwind area. Results show that the Nusselt number for the case with conical-strip inserts is only 53–56% of that for the case with conical-ring inserts, the friction factor for the former is merely 4–6% of that for the latter, and thus the thermo-hydraulic performance factor can be enhanced by 36–61% if replacing conical-ring inserts with conical-strip inserts for turbulent flow within Re range 5000–25,000. In addition, a comparative study of the short conical strip and the T-type conical strip, which can be regarded as boundary-cutting and core-cutting to the normal conical strip respectively, is performed. Results indicate that the flow resistance can be reduced by both methods. However, a weakened heat transfer and a worsened thermo-hydraulic performance are obtained for the case with boundary-cutting strip inserts. In contrast, the case with T-type core-cutting conical-strip inserts, if a suitable core-cutting size is taken, shows a better thermo-hydraulic performance as compared with the case with normal conical-strip inserts when the Reynolds number is relatively large. Effects of the pitch between neighboring strips have also been examined. It is found that smaller pitch leads to higher heat transfer rate whereas the flow resistance is increased. A moderate pitch between conical strips is beneficial to the overall thermo-hydraulic performance. [Copyright &y& Elsevier]

Abstract: Twisted tape is a widely used technique for heat transfer enhancement. In the present paper, we proposed a center-cleared twisted tape aiming at acxhieving good thermohydraulic performance. A comparative study between this type and the short-width twisted tape was performed numerically in laminar tubular flows. The computation results demonstrated that the flow resistance can be reduced by both methods; however, the thermal behaviors are very different from each other. For tubes with short-width twisted tapes, the heat transfer and thermohydraulic performance are weakened by cutting off the tape edge. Contrarily, for tubes with center-cleared twisted tapes, the heat transfer can be even enhanced in the cases with a suitable central clearance ratio. The thermal performance factor of the tube with center-cleared twisted tape can be enhanced by 7–20% as compared with the tube with conventional twisted tape. All these demonstrated that the center-cleared twisted tape is a promising technique for laminar convective heat transfer enhancement. [Copyright &y& Elsevier]

Abstract: The pool boiling heat transfer performance of refrigerant R134a on single horizontal tube surfaces sintered with open-celled copper foam was studied at T s =6°C and P s =3.62bar. Three pore density values: 40, 80, and 130 PPI; two porosity values: 90% and 97%; and two thickness values: 1.6 and 2.5mm, characterize the 12 studied tubes. The nucleate boiling data and heat transfer coefficients were obtained. The influences of pore density, porosity, and foam thickness on heat transfer performance were investigated as well. The foam provides activated nucleation sites; however, it also causes transport resistance for the bubbles within the foam to move from the active nucleation sites to the surface. Performance is dependent on whichever of these two prevails in the heat transfer process. The foam-coated tubes show significant performance enhancement compared with plain tubes at heat fluxes below 30kW/m2. However, a sharp reduction in heat transfer coefficient is encountered for 130 PPI at larger heat fluxes. Tubes coated with thin foam and high porosity, e.g., pore density: 80 PPI, porosity: 97%, and thickness: 1.6mm, exhibit a comparatively superior heat transfer performance. The current experimental data provide useful information for the design of highly effective boiling surfaces. [Copyright &y& Elsevier]