Maximum Of Heat Transfer Coefficient Research Articles

Effect of steel-wires and paraffin composite phase change materials on the heat exchange and exergetic performance of salt gradient solar pond

The addition of Phase Change Materials (PCM) to the salt gradient solar pond can significantly increase the heat capacity of the solar pond. Paraffin wax has a suitable range of phase transitions temperature and is feasible to be applied in solar pond. However, the poor thermal conductivity of paraffin is detrimental to thermal storage and release. In this study, the heat transfer behavior for the composite PCM of paraffin and steel wires has been studied firstly, the results shows that adding steel wires in paraffin improve the heat transfer performance obviously. With 10wt% adding percent, the maximum of heat transfer coefficient is 18.17 W/m2 °C−1 with increment of 18.84 % in melting process; and the maximum is 21.59 W/m2 °C−1 with increment of 20.88% during freezing process. Secondly, experiment study has been conducted in an experimental solar pond, with 20 PCM capsule tubes added, adding RT 50 and RT 60 made the day–night temperature differences 2.87 °C and 2.53 °C smaller than that of without any PCM conditions. In this study, adding RT 50 and RT 60 in solar pond achieve 9.34% and 14.13% exergy increments respectively. The study suggested that adding PCM to solar pond improves the quality of the heat energy of solar pond, and further extends the heat application scope of solar pond.

Centerline heat transfer coefficient distributions of synthetic jets impingement cooling

The paper presents the results of experimental investigations of heat transfer coefficients, velocity and pressure distributions of synthetic jets for two cases: constant Reynolds number and variable Stokes number and vice versa. Measurements were made for synthetic jet Reynolds number in the range of Re=5440–11,475, dimensionless stroke length L0=7.49–38.5 and dimensionless jet-to-plate spacing x/d = 0–20. The synthetic jet time-averaged and momentum velocity as well as the pressure for each case are presented. The centerline turbulence intensity and the Euler number are also reported. The centerline heat transfer coefficient distributions are compared with the distributions of the various velocity and pressure parameters. A new correlation between the local Nusselt number and the local Reynolds number is presented. Under synthetic jet impingement cooling the maximum of the stagnation point heat transfer coefficient appears mostly at axial distance of x/d ≈ 5. The location of the maximum Nusselt number at x/d ≈ 5 is caused by the modification of the heat transfer distribution due to the recirculation of hot air. With the decrease of the heat power released into the control volume by the impingement plate (heater) this modifying effects becomes less pronounced thus the maximum of the heat transfer coefficient shifts towards the location of about x/d = 2. The maxima of various flow parameters are generally observed at about x/d = 2 what was also reported by other independent researchers. Thus, it may be concluded that the purely mechanical phenomena does not full coincide with thermal ones in the case of synthetic jet impingement cooling.

Experimental identification and a model of a local heat transfer coefficient for water – Air assisted spray cooling of vertical low conductivity steel plates from high temperature

A local heat transfer coefficient (HTC) has been determined for six cases of vertical plates cooling by the water-air assisted sprays. Full cone 60° nozzle operating under pressure of 0.1, 0.25 and 0.4 MPa has been employed. The plates have been made from the Inconel alloy and stainless steel. The Inconel alloy differs to some extend in thermophysical properties from stainless steel. Plate temperature has been measured with thermocouples at 25 points distributed over 1/4 of the cooled surface. The inverse solutions have given HTC as function of surface coordinates and time. The HTC maximum has varied from 3 to 10 kW/(m2∙K) over water stream zone. The critical heat flux ranging from 600 kW/m2 to 2.5 GW/m2 has been obtained depending on pressure, location and plate material. Based on a large set of HTC a model of local HTC as function of pressure, surface temperature and distance from the stagnation point has been developed. The HTC model depends on 15 coefficients which has been determined for Inconel and stainless steel. The HTC model has been validated. Energy extracted from the plate surface and temperature distributions predicted by the model have been compared to data obtained from inverse solutions and temperature measurements. It has been concluded that the model accuracy is sufficient for simulations of water spray cooling of low conductivity steels from 1200 °C to 50 °C.

Heat transfer of supercritical carbon dioxide flowing in a rectangular circulation loop

Experiments were conducted to investigate the heat transfer of supercritical carbon dioxide flowing in a rectangular circulation loop. The effects of heat flux, pressure, buoyancy force on heat transfer of supercritical carbon dioxide were carefully discussed. A heat transfer correlation for supercritical fluid was developed based on experimental data. Distinct heat transfer enhancement and heat transfer deterioration were observed in the present experiments. The maximum of heat transfer coefficient decreased with the increase of heat flux and pressure. Noticeable heat transfer deterioration was only observed in conditions of buoyancy parameter higher than 2.2 × 10−5, Tw,int/Tpc higher than 1.4, Tb/Tpc less than 0.95. The mechanisms of the effects of heat flux, pressure and buoyancy force on heat transfer of supercritical carbon dioxide were discussed in detail. Results indicated that the steep gradient of physical property over the tube cross-section is the key reason for heat transfer deterioration. The new heat transfer correlation showed reasonably satisfactory prediction performance.

Numerical Study of Turbulent Heat Transfer in Annular Pipe with Sudden Contraction

Turbulent heat transfer to air flow in annular pipe with sudden contraction numerically studied in this paper. The k-ε model with finite volume method used to solve continuity, moment and energy equations. The boundary condition represented by uniform and constant heat flux on inner pipe with range of Reynolds number varied from 7500 to 30,000 and contraction ratio (CR) varied from 1.2 to 2. The numerical result shows increase in local heat transfer coefficient with increase of contraction ratio (CR) and Reynolds number. The maximum of heat transfer coefficient observed at contraction ratio of 2 and Reynolds number of 30,000 in compared with other cases. Also pressure drop coefficient noticed rises with increase contraction ratio due to increase of recirculation flow before and after the step height. In contour of velocity stream line can be seen that increase of recirculation region with increase contraction ratio (CR).

Heat and Mass Coupled Transfer Combined with Freezing Process in Building Exterior Envelope

In severe cold area, the exterior layers of envelope usually experience seasonal freezing /thawing due to cold climate in winter. However, it can lead to severe problems especially in the newly-completed building. In order to analyze the drying of envelope on building initial use, the heat and moisture coupled transfer of building envelope in severe cold area Harbin, China was simulated with the modeling method developed. The modeled result was analyzed. It is concluded that the drying rate of the newly-completed building envelope is significantly high in the first year, especially in the first few months. Insulation performance of the wall in the first winter is most serious due to the high initial moisture and freezing ice content in insulation layer. For the simulated envelope, the freezing of the moisture content especially that in insulation layer had notable effect on heat transfer coefficient (thermal resistance), and the maximum of heat transfer coefficient of the modeled envelope in the first year winter is about 7% higher than that in the tenth year, which is taken as final hygral state.

Shallow fluidised-bed heat transfer

Shallow fluidised-beds have great potential as heat exchangers, since the pressure drop incurred is low enough not to require the use of large fans. An investigation of shallow-bed heat transfer is reported here and the observations are compared with established deep-bed experience. Several peculiarities were found and are discussed and the heat transfer data are compared quantitatively with deep-bed correlations. The prediction of the fluidising velocity at which the heat transfer coefficient maximum is obtained is considered in some detail, since this is an important consideration in heat exchanger design. A simple hypothesis based upon the two-phase theory of fluidisation is proposed as an alternative to the present, inaccurate empirical approach.