A heat transfer correlation for transient vapor uptake of powdered adsorbent embedded onto the fins of heat exchangers

Experimental study of local air-side heat transfer coefficient on real-scale heat exchanger fins by employing an absorption-based mass transfer method

Flow Boiling Pressure Drop and Heat Transfer of Refrigerants in Multi-microchannel Evaporators under Steady and Transient States

Transient natural convection heat transfer of liquid D-mannitol on a horizontal cylinder

Experimental evaluation of local instantaneous heat transfer characteristics in the combustion chamber of air-cooled direct injection diesel engine

ABSTRACTThree-dimensional finite element models with consideration of shrinkage and irregular shape were developed to estimate the relationships among the transient heat and mass transfer coefficients, the transient water diffusivity, and the temperature and moisture content of the red kidney beans being dried under different drying conditions. An equation was developed to calculate the transient mass transfer coefficient using the measured time–moisture content data. This calculated transient mass transfer coefficient was further used to calculate the transient heat transfer coefficient. To verify the predicted temperature on the surface of the red kidney beans, surface temperature was measured using a handhold infrared thermometer. These measured temperature and time–moisture content data were used to determine the transient water diffusivity using the least square method when the red kidney bean kernel experienced a shrinkage during drying. Strong relationship among the transient heat and mass transfer coefficients, the water diffusivity, and the ratio of the transient heat and mass transfer coefficients was revealed. This relationship could be used to predict temperature and moisture content of the red kidney beans during the entire drying period. The Lewis number = 27, and the ratio of the transient heat over mass transfer coefficients was 10765 J m−3 k−1 at 30 and 40°C, and 10729 J m−3 k−1 at 50°C. Shrinkage did not significantly influence the value of the estimated transient water diffusivity.

Effects of frost growth on louvered folded fins of microchannel heat exchangers on the time-dependent air side convective heat transfer coefficient

Transient forced convection heat transfer due to exponentially increasing heat input to a heater is important as a database for safety assessment of the transient heat transfer process in a Very High Temperature Reactor (VHTR). The knowledge of heat transfer enhancement using a heater with twisted configuration is also important for the high performance design of intermediate heater exchanger (IHX) in VHTR system. In this study, forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a short thin twisted plate with various helix angles was experimentally studied. A forced convection heat transfer experimental apparatus was used to measure the experimental data. The test heater was mounted horizontally along the center part of a circular test channel. Twisted plates were made of thin platinum plate with a thickness of 0.1 mm and width of 2 mm and 4 mm. The heat generation rates of the heater were controlled and measured by a heat input control system. The heat generation rate, Q̇, was raised with exponential function, Q̇ = Q0exp(t/τ). Where, t is time, and τ is period of heat generation rate. The mean temperature of the test heater was measured by resistance thermometry. The heat flux was obtained by the energy conservation equation. The test heater surface temperature was calculated from heat conduction equation of the heater. The transient heat transfer experimental data were measured for the periods ranged from 80 ms to 17 s and at a gas temperature of 303 K under 500 kPa. The flow velocities ranged from 4 m/s to 10 m/s. In the experiments, the twisted plates with different width were tested. The surface temperature and heat flux are increasing exponentially with the time. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than about 1 s. The heat transfer coefficients for total length of the twisted plate were compared with the values of flat plate which has the same width and thickness with the twisted one. The local mean heat transfer coefficients have been tested as well. The heat transfer coefficients of twisted plate are about 10% for 2 mm-width one and15% for 4 mm-width one higher than those of flat plate with same width at the quasi-steady state. And also, the heat transfer coefficients for the first half pitch are 24% higher than that for the total length of the same twisted plate. Therefore, an enhancement in the heat transfer coefficient for the twisted plate was clarified.

This paper presents a computational fluid dynamics (CFD) study of a building-integrated photovoltaic thermal (BIPV∕T) system, which generates both electricity and thermal energy. The heat transfer in the BIPV∕T system cavity is studied with a two-dimensional CFD model. The realizable k‐ε model is used to simulate the turbulent flow and convective heat transfer in the cavity, including buoyancy effect and long-wave radiation between boundary surfaces is also modeled. A particle image velocimetry (PIV) system is employed to study the fluid flow in the BIPV∕T cavity and provide partial validation for the CFD model. Average and local convective heat transfer coefficients are generated with the CFD model using measured temperature profile as boundary condition. Cavity temperature profiles are calculated and compared to the experimental data for different conditions and good agreement is obtained. Correlations of convective heat transfer coefficients are generated for the cavity surfaces; these coefficients are necessary for the design and analysis of BIPV∕T systems with lumped parameter models. Local heat transfer coefficients, such as those presented, are necessary for prediction of temperature distributions in BIPV panels.

The heat transfer from surfaces may in general be enhanced by increasing the heat transfer coefficient between a surface and its surroundings, and by increasing the heat transfer area of the surface, or by both. In most cases the heat transfer is concerned with the application of artificial surface roughness techniques as a turbulence promoter for convective heat transfer enhancement. To measure the transient and local convective heat transfer coefficient of an artificial roughened duct using liquid crystal thermography (LCT) technique, an experimental setup have been designed and developed. The aspect ratio of duct was maintained as three with relative roughness height ratio (e/D) of 0.13 and angle of attack is 900 for the range of Reynolds number from 7000 to 17500.

Unsteady Evaporation of Water from Wire Mesh Structures at Sub-Atmospheric Pressures

Local heat transfer coefficients at the inlet of an annular flow passage

Local two-phase heat transfer from arrays of confined and submerged impinging jets

Combined cycle units have become very popular in recent years as a source of power generation. Such units have a gas turbine as the topping cycle and a steam turbine as the bottoming cycle and can reach combined cycle efficiencies as high as 60%. The exhaust from the gas turbine is passed through a heat exchanger in which steam is generated for the steam turbine. This combined arrangement makes it less polluting as well. An important element of a combined cycle power plant is the steam turbine, which is the subject of this paper. Improvements to the design of advanced steam turbines require an improved understanding of the heat transfer within the various components of the unit. Physics-based ANSYS models for typical GE high pressure and intermediate pressure units have been developed. Components such as the rotor, diaphragm, and shells have been analyzed. The boundary conditions were derived from full-load, steady state flow analyses, steam turbine performance code outputs and computational fluid dynamics (CFD) analyses to develop normalized (non-dimensional) local flow conditions, with the normalizing parameters based on key cycle parameters. These normalized local flow conditions and cycle parameters were then used to define local transient boundary temperatures and heat transfer coefficients for input to the thermal ANSYS models. Transient analyses of components were performed. The results were compared with temperature measurements taken during the complete cycle of an operational steam turbine to validate and improve the methodology, and were applied to structural models of the components to predict their thermal growth and the net impact on the clearance between the rotor and diaphragms and other secondary flow paths in the steam turbine, including the packing seals. This paper will focus on the thermal modeling of a typical steam turbine. It will discuss the process used (summarized above) and the basic equations employed in the analyses. Results will be compared with shell temperature measurements obtained during the start up of a steam turbine in the field. Implications of the thermal results on power systems operation will be discussed. Plans for future improvements will be presented.

Experimental study on transient heat transfer characteristics during the dropping of containment pressure

The passive containment cooling system (PCCS) has been applied in the new generation nuclear power plant. Previous studies mainly focused on steady-state heat transfer, but the actual heat transfer of PCCS is the transient process. Thus, investigating the transient heat transfer during the containment pressure dropping is necessary. The present study proposes a time-average condensation heat transfer coefficient (time-average condensation HTC) to characterize the intensity of transient condensation heat transfer. The time-average condensation HTC is defined as the heat absorbed per unit heat transfer area and per unit wall subcooling in a unit time. Experiments were performed to research the effect of initial gas-mixture pressure and air mass fraction on transient HTC. The result showed that the more significant gas-mixture pressure and lower air mass fraction could promote the transient-state heat transfer, especially the low air mass fraction. Based on the experimental result, a detailed empirical correlation for the time-average heat transfer coefficient is developed with an error of ±20%. Another simplified empirical correlation that only includes air mass fraction is also proposed to predict the transient heat transfer roughly. Besides, research on self-sustaining stability is also conducted to evaluate the stable operation characteristic of PCCS. The system can respond quickly and then run to a new steady state after interference during the long-term operation of PCCS. The phenomenon above implies that PCCS has good self-sustaining stability.