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21 
Heat transfer in a packed bed using a fluid near its critical point for solar energy applicationsAlChalaby, K. A. J. January 1987 (has links)
No description available.

22 
The mechanism of dropwise condensation of steamFang, ChungChih January 1949 (has links)
The present investigation can be divided into two parts: (a) experiments made to examine the mechanism of dropwise condensation of steam with particular reference to the stability of drop promoting surfaces as affected by the material of cooled surface, the drop promoter, the surface finish, the rate of heat transmission, and the presence of noncondensable gas. and (b) a theoretical analysis of the beat transmission through individual droplets, the transient heat transfer through exposed areas, the statistical study of drop size distribution, and the estimation of steam side coefficient. An apparatus was developed to examine qualitatively,the behaviour of drop promoting surfaces on a small scale. It is considered that sufficient evidence was found to show that steam in contact with a cooled surtace condenses as a thin liquid film which later breaks into droplets. surfaces treated to give dropwise condensation deteriorate into mixed condensation in due time, and the duration tor which a treated surface maintains dropwise condensation varies between a few hours to several days, depending on many factors among which· the presence of noncondensable gas must not be overlooked. An approximation to the heat transmission through individual droplets has been worked out with assumed heat flow lines. The result, checked by the relaxation method. is correct within . + 10%. An analysis Of the transient heat transfer through exposed areas was made neglecting the increasing resistance of any accumulating liquid. The drop size distribution was analyzed tor one drop promoting surface at three different heat transmission rates. Based on this drop size distribution, the heat transmission through the drops was estimated by assuming they were held at rest on a cooled surface conducting heat under a steady state. . The estimated coefficient comes within the range or experimental results of many investigators.

23 
Characterization of thermophysical properties and forced convective heat transfer of polyalphaolefin (PAO) nanofluids.Nelson, Ian Carl 15 May 2009 (has links)
Colloidal solvents, containing dispersed nanometer (~1100 nm) sized particles, are categorized as nanofluids. With the growing heat loads in engineering systems that exceed the current technological limits, nanofluids are considered as an attractive option for more efficient heat removal for thermal management applications. Recent results reported in the literature show that the thermophysical properties of coolants are enhanced considerably when seeded with very minute concentrations of nanoparticles. Hence, nanofluids research has provoked interest in thermal management applications. The convective heat transfer characteristics of nanofluids are reported in this study. Exfoliated graphite nanoparticles were dispersed in polyalphaolefin (PAO) at concentrations of 0.3% and 0.6% (by weight). The heat flux into a convective cooling apparatus was monitored and the results for nanofluid and the base fluid are presented. Thermophysical properties of the nanofluid were measured and compared with the base fluid. The thermophysical properties of the fluid are observed to increase with the addition of the nanoparticles. The specific heat of nanofluid was increased by ~50% compared to PAO. The thermal diffusivity was enhanced by ~400% compared to PAO. The viscosity of the nanofluid was enhanced by 101000 times compared to PAO. The viscosity of the nanofluid was observed to increase with temperature while the viscosity of PAO decreases with temperature. The convective heat flux was enhanced by the nanofluids by up to ~8 % for experiments performed at different heat inputs. The experimental results show that the convective heat transfer enhancement potentially results from the precipitation of nanoparticles on the heated surface and results in enhanced heat transfer surfaces (“nanofins”).

24 
Characterization of thermophysical properties and forced convective heat transfer of polyalphaolefin (PAO) nanofluids.Nelson, Ian Carl 15 May 2009 (has links)
Colloidal solvents, containing dispersed nanometer (~1100 nm) sized particles, are categorized as nanofluids. With the growing heat loads in engineering systems that exceed the current technological limits, nanofluids are considered as an attractive option for more efficient heat removal for thermal management applications. Recent results reported in the literature show that the thermophysical properties of coolants are enhanced considerably when seeded with very minute concentrations of nanoparticles. Hence, nanofluids research has provoked interest in thermal management applications. The convective heat transfer characteristics of nanofluids are reported in this study. Exfoliated graphite nanoparticles were dispersed in polyalphaolefin (PAO) at concentrations of 0.3% and 0.6% (by weight). The heat flux into a convective cooling apparatus was monitored and the results for nanofluid and the base fluid are presented. Thermophysical properties of the nanofluid were measured and compared with the base fluid. The thermophysical properties of the fluid are observed to increase with the addition of the nanoparticles. The specific heat of nanofluid was increased by ~50% compared to PAO. The thermal diffusivity was enhanced by ~400% compared to PAO. The viscosity of the nanofluid was enhanced by 101000 times compared to PAO. The viscosity of the nanofluid was observed to increase with temperature while the viscosity of PAO decreases with temperature. The convective heat flux was enhanced by the nanofluids by up to ~8 % for experiments performed at different heat inputs. The experimental results show that the convective heat transfer enhancement potentially results from the precipitation of nanoparticles on the heated surface and results in enhanced heat transfer surfaces (“nanofins”).

25 
Simulation of Gaseous Flow in a MicrochannelWang, YiTing 07 July 2003 (has links)
A numerical prediction using the Direct Simulation Monte Carlo method (DSMC)has been performed on low speed gas flows through a short parallel plate microchannel(L/Dh=6). Computations were carried out for nitrogen, argon, and helium gas. Micro pressure driven flows are simulated with the inlet value of the Knudsen numbers ranging from 0.09 to 0.2. The effects of varying pressure, wall temperature, inlet flow and gas transport properties on the wall heat transfer, pressure and velocity distribution were examined. Friction factors and heat transfer from the channel were also calculated and compared with those of previous studies. Finally, the averaged Nusselt number was correlated in a simple form of the averaged Peclet number and Knudsen number in the transition flow regime.

26 
Aspect ratio effect on heat transfer in rotating twopass rectangular channels with smooth walls and ribbed wallsFu, WenLung 29 August 2005 (has links)
This study experimentally investigates the effects of rotation, the buoyancy force,
and the channel aspect ratio on heat transfer in twopass rotating rectangular channels.
The experiments are conducted with two surface conditions: smooth walls and 45??
angled ribbed walls. The channel aspect ratios include 4:1, 2:1, 1:1, 1:2 and 1:4. Four
Reynolds numbers are studied: 5000, 10000, 25000 and 40000. The rotation speed is
fixed at 550 rpm for all tests, and for each channel, two channel orientations are studied:
90?? and 45?? or 135??, with respect to the plane of rotation. Rib turbulators are placed on
the leading and trailing walls of the channels at an angle of 45?? to the flow direction. The
ribs have a 1.59 by 1.59 mm square cross section, and the rib pitchtoheight ratio (P/e)
is 10 for all tests.
The effects of the local buoyancy parameter and channel aspect ratio on the
regional Nusselt number ratio are presented. Pressure drop data are also measured for
both smooth and ribbed channels in rotating and nonrotating conditions. The results
show that increasing the local buoyancy parameter increases the Nusselt number ratio on
the trailing surface and decreases the Nusselt number ratio on the leading surface in the
first pass for all channels. However, the trend of the Nusselt number ratio in the second
pass is more complicated due to the strong effect of the 180?? turn. Results are also
presented for this critical turn region of the twopass channels. In addition to these
regions, the channel averaged heat transfer, friction factor, and thermal performance are
determined for each channel. With the channels having comparable Nusselt number
ratios, the 1:4 channel has the superior thermal performance because it incurs the least
pressure penalty. In this study, the author is able to systematically analyze, correlate, and
conclude the thermal performance comparison with the combination of rotation effects
on five different aspect ratio channels with both smooth walls and rib turbulated walls.

27 
Effect of rib spacing on heat transfer and friction in a rotating twopass rectangular (AR=1:2) channelLiu, YaoHsien 30 October 2006 (has links)
The research focuses on testing the heat transfer enhancement in a channel for
different spacing of the rib turbulators. Those ribs are put on the surface in the two pass
rectangular channel with an aspect ratio of AR=1:2. The cross section of the rib is
1.59 x 1.59 mm. Those ribs are put on the leading and trailing walls of the channel with
the angle of flow attack to the mainstream of 45ÃÂ°. The rotating speed is fixed at 550RPM
with the channel orientation at ÃÂ²=90ÃÂ°. Air is used as the coolant through the cooling
passage with the coolanttowall density ratio ( ÃÂ ÃÂ Ã¢ÂÂ ) maintained around 0.115 in the
first pass and 0.08 in the second pass. The Reynolds numbers are controlled at 5000,
10000, 25000, and 40000. The rib spacingtoheight ratios (P/e) are 3, 5, 7.5, and 10. The
heat transfer coefficient and friction factor are measured to determine the effect of the
different rib distributions. Stationary cases and rotational cases are examined and
compared. The result shows that the highest thermal performance is P/e=5 for the
stationary case and P/e=7.5 for the rotating case.

28 
Heat transfer enhancement for turbulent flow through blockages with elongated holes in a rectangular channelLee, Yonghee 17 September 2007 (has links)
In this thesis, turbulent forced convective heat transfer downstream of blockages
with elongated holes in a rectangular channel was studied.
The rectangular channel has a widthtoheight ratio of 12:1. The blockages have
the same cross section as that of the channel. The diameter of all elongated holes of the
blockages is three quarters of the channel height. The blockages are classified into two
different types with two different holetoblockage area ratios (ratio of total crosssectional
area of holes to crosssectional surface area of the blockage) of 0.5 or 0.6. For
each holetoblockage area ratio, the blockages are again subdivided into three different
cases using three different aspect ratios (holewidthtoheight ratio) which are
determined by the number of holes four, six, and eight holes per blockage.
Experiments for total six different cases of blockages were performed under a uniform
wall temperature condition (50C). The experiments were conducted at three different
Reynolds numbers of about 7,000, 12,000, and 17,000, respectively. Three copper plate
heaters with twenty one embedded thermocouples were used to measure the average heat
transfer on the surface of channel walls between two consecutive blockages.
Results from this study showed that the blockages with elongated holes enhance
the average heat transfer by up to 5.06 and 4.08 times that by fully developed turbulent
flow through a smooth channel at the same Reynolds numbers for small and large holeto
blockage area ratios, respectively. The friction factor ratios for small and large holeto
blockage area ratios of the blockages reached 345 and 89 times, respectively, that by
fully developed turbulent flow through a smooth channel at the same Reynolds numbers.
TP (Thermal Performance) values varied from 0.65 to 1.11 depending on cases.
According to the results, Case L2, which has six elongated holes and holetoblockage
area ratio of 0.6, is the best option from the TP point of view. But Case S2,
which also had six elongated holes and holetoblockage area ratio of 0.5, can be an
alternative when more weight should be put on the heat transfer enhancement than TP
value.

29 
A numerical study of vorticityenhanced heat transferWang, Xiaolin 21 September 2015 (has links)
In this work, we have numerically studied the effect of the vorticity on the enhancement of heat transfer in a channel flow. In the first part of the work, we focus on the investigation of a channel flow with a vortex street as the incoming flow. We propose a model to simulate the fluid dynamics. We find that the flow exhibits different properties depending on the value of four dimensionless parameters. In particularly, we can classify the flows into two types, active and passive vibration, based on the sign of the incoming vortices. In the second part of the work, we discuss the heat transfer process due to the flows just described and investigate how the vorticity in the flow improves the efficiency of the heat transfer. The temperature shows different characteristics corresponding to the active and passive vibration cases. In active vibration cases, the vortex blob improves the heat transfer by disrupting the thermal boundary layer and preventing the decay of the wall temperature gradient throughout the channel, and by enhancing the forced convection to cool down the wall temperature. The heat transxfer performance is directly related to the strength of the vortex blobs and the background flow. In passive vibration cases, the corresponding heat transfer process is complicated and varies dramatically as the flow changes its properties. We also studied the effect of thermal parameters on heat transfer performance. Finally, we propose a more realistic optimization problem which is to minimize the maximum temperature of the solids with a given input energy. We find that the best heat transfer performance is obtained in the active vibration case with zero background flow.

30 
An experimental and numerical invetigation of laminar and turbulent natural convection in vertical parallelplate channelsYilmaz, Turgut January 1997 (has links)
Laminar and turbulent natural convection heat transfer and fluid flow processes in asymmetrically heated vertical parallel flat plate channels have been investigated both experimentally and numerically. The experimental part of the investigation was carried out using a fibre optic LaserDoppler Anemometer (LDA) together with a temperature probe and data acquisition system. The numerical analysis was done using the computational fluid dynamics (CFD) code PHOENICS. The laminar natural convection flow case was examined for a uniform heat flux (UHF) heating mode while the turbulent flow case was examined considering both UHF and uniform wall temperature (UWT) heating modes. Small and large scale channels were constructed to perform laminar and turbulent flow experiments respectively. The channels were formed by a heated wall, an opposing unheated perspex or glass wall and side walls. Velocity profile and time history data along the channel and temperature profiles at channel exit were recorded for the laminar flow case. In the turbulent flow case of UWT heating mode mean and turbulent velocity data and mean temperature data at the channel exit were collected. For the UHF heating mode turbulent flow case, however, both mean and turbulent velocity and temperature data were recorded. The main objective of the experiments was to obtain data by which the numerical analysis could be supported since not enough experimental data is available, especially for the turbulent flow case. The numerical analysis of the laminar flow situation was performed with the standard features of PHOENICS. The turbulent flow case was examined by building into PHOENICS the codes for four different Low Reynolds Number kƐ turbulence models. The grid pattern was optimised for a test case and employed for final computations. Due to the lack of experimental data regarding the location of transition to turbulent flow, computations were done by introducing a level of turbulence at the inlet and solving the governing equations for turbulent flow throughout the channel. The results of measurements justified this approach. The results of numerical and experimental analysis of laminar flow showed good agreement confirming that the numerical method was capable of predicting the flow with good accuracy. Turbulent flow experiments exhibited the characteristics of a developing turbulent channel flow. Low amplitude, high frequency velocity fluctuations were observed close to the channel inlet with higher amplitudes downstream and almost similar fluctuation patterns in the uppermost region of the channel suggesting fully developed turbulence. The temperature data indicated intermittent bursts of high frequency temperature fluctuations near the inlet and continuous high frequency temperature fluctuations of increasing, amplitude downstream. Numerical results of the turbulent flow case have produced good agreement with experiments. The solutions were most sensitive to the level of turbulence introduced at the channel inlet. Reasonable grid independent solutions were obtained for grids greater than 60x60 for most of the turbulence models considered. Velocity and temperature profiles obtained from experiments and numerical analysis are presented for both flow cases. Typical time histories of velocity and temperature at different channel heights and crossstream locations are presented. From the numerical and experimental results correlations were produced for Nusselt number, and Reynolds number as functions of Rayleigh number, for the UWT heating mode of turbulent flow.

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