Investigation of Circumferential Variation of Heat Transfer Coefficients During In-Tube Evaporation for R-22 and R-407C Using Liquid Crystal

2002 ◽  
Vol 124 (5) ◽  
pp. 845-853 ◽  
Author(s):  
Seok Ho Yoon ◽  
Min Soo Kim
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Cross Sections ◽  
Outer Wall ◽  
Processing Technique ◽  
Image Processing Technique ◽  
Measured Temperature ◽  
Thermochromic Liquid Crystal ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

Heat transfer coefficients during evaporation in a horizontal smooth tube for R-22 and R-407C (R-32/125/134a, 23/25/52 wt.%) have been measured using thermochromic liquid crystal. Focus has been put on the circumferential variation of heat transfer coefficients at several cross-sections of the test tube with inner diameter of 11.3 mm for several vapor qualities of refrigerant. The inner wall temperatures were calculated by one dimensional heat conduction equation from the measured outer wall temperatures, which were obtained using an image processing technique with thermochromic liquid crystal (TLC). The relation between measured temperature and color information (Red-Green-Blue values) of thermochromic liquid crystal was calibrated by a neural network method. Results show that circumferential variation of heat transfer coefficients for R-22 is quite large with the highest heat transfer coefficient at the top of the tube. For zeotropic mixture of R-407C, similar trend has been observed with less difference between the heat transfer coefficients at the top and bottom than that of R-22.

Improved Accuracy in Jet Impingement Heat Transfer Experiments Considering the Layer Thicknesses of a Triple Thermochromic Liquid Crystal Coating

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Alexandros Terzis ◽  
Stavros Bontitsopoulos ◽  
Peter Ott ◽  
Jens von Wolfersdorf ◽  
Anestis I. Kalfas
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Thickness Gauge ◽  
Thermochromic Liquid Crystal ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Paint Thickness

This paper examines the applicability of a triple layer of thermochromic liquid crystals (TLCs) for the determination of local heat transfer coefficients using the transient liquid crystal (LC) technique. The experiments were carried out in a narrow impingement channel, typically used for turbine blade cooling applications. Three types of narrow bandwidth LCs (1 °C range) of 35 °C, 38 °C, and 41 °C were individually painted on the target plate of the cooling cavity and the overall paint thickness was accurately determined with an integral coating thickness gauge. The 1D transient heat conduction equation is then implicitly solved for each individual TLC layer on its realistic depth on the painted surface. Local heat transfer coefficients are therefore calculated three times for the same location in the flow improving the measurement accuracy, especially at regions where the LC detection times are too short (stagnation points) or too long (wall-jet regions). The results indicate that if multiple LC layers are used and the paint thickness is not considered, the heat transfer coefficients can be significantly underestimated.

MEMS Impinging-Jet Cooling

2000 ◽  
Author(s):  
Qiao Lin ◽  
Shuyun Wu ◽  
Yin Yuen ◽  
Yu-Chong Tai ◽  
Chin-Ming Ho
Keyword(s):  
Heat Transfer ◽  
Heat Exchangers ◽  
Jet Impingement ◽  
Impinging Jet ◽  
Impinging Jets ◽  
Measured Temperature ◽  
Transfer Coefficients ◽  
Element Analysis ◽  
Heat Transfer Coefficients ◽  
Jet Cooling

Abstract This paper presents an experimental investigation on MEMS impinging jets as applied to micro heat exchangers. We have fabricated MEMS single and array jet nozzles using DRIE technology, as well as a MEMS quartz chip providing a simulated hot surface for jet impingement. The quartz chip, with an integrated polysilicon thin-film heater and distributed temperature sensors, offers high spatial resolution in temperature measurement due to the low thermal conductivity of quartz. From measured temperature distributions, heat transfer coefficients are computed for single and array micro impinging jets using finite element analysis. The results from this study for the first time provide extensive data on spatial distributions of micro impinging-jet heat transfer coefficients, and demonstrate the viability of MEMS heat exchangers that use micro impinging jets.

Using Servo-Drive Presses to Determine the Effect of Blank Holder Pressure on Temperature Change in Warm Forming of Sheet

2011 ◽  
Vol 133 (6) ◽  
Author(s):  
Serhat Kaya
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Temperature Change ◽  
Warm Forming ◽  
Measured Temperature ◽  
Transfer Coefficients ◽  
Servo Drive ◽  
Blank Holder ◽  
Heat Transfer Coefficients ◽  
Deform 2D

Heat transfer coefficient (HTC) is one of the most important and difficult-to-obtain parameter in high temperature environment. Contact pressure and workpiece surface roughness are among important parameters that affect the heat transfer in elevated temperature forming of sheets. In this study, HTCs are investigated experimentally by using a servo-drive press. With the flexibility that the servo-drive press offers, effect of various blank holder pressures on temperature change is determined. Before and after surface roughness conditions of aluminum and magnesium (from two different manufacturers) alloy sheets are compared. Experimental setup was modeled using deform 2d, and measured temperature curves were compared with the finite element analysis (FEA) predictions and a window of heat transfer coefficients were determined for warm forming of sheets. Determined heat transfer coefficients were implemented in a nonisothermal deep drawing FE model in deform 2d and results were compared with experiments. Good agreement was obtained between FEA predictions and experiments.

Calibration of External Heat Transfer Coefficients During Cooling of a Partially-Filled Water Tank Using Measured Temperature-Time Data

2018 ◽  
Author(s):  
Vishal Ramesh ◽  
Sandip Mazumder ◽  
Gurpreet Matharu ◽  
Dhaval Vaishnav ◽  
Syed Ali ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Measured Temperature ◽  
Water Tank ◽  
Time Data ◽  
Transfer Coefficients ◽  
Ansys Fluent ◽  
Heat Transfer Coefficients ◽  
Computational Fluid Dynamics Cfd ◽  
Temperature Time

A combined Computational Fluid Dynamics (CFD) and experimental approach is presented to determine (calibrate) the external convective heat transfer coefficients (h) around a partially-filled water tank cooled in a climactic chamber. A CFD analysis that includes natural convection in both phases (water and air) was performed using a 2D-axisymmetric tank model with three prescribed average heat transfer coefficients for the top, side and bottom walls of the tank. The commercial CFD code ANSYS-Fluent™, along with User-Defined Functions (UDFs), were utilized to compute and extract temperature vs. time curves at five different thermocouple locations within the tank. The prescribed h values were then altered to match experimentally obtained temperature-time data at the same locations. The calibration was deemed successful when results from the simulations exhibited match with experimental data within ±2°C for all thermocouples. The calibrated h values were finally used in full-scale 3D simulations and compared to the experimental data to test their accuracy. Predicted 3D results were found to agree with experimental results within the error of the calibration, thereby lending credibility to the overall approach.

Effect of Rib Configuration and Lateral Ejection on a High Aspect Ratio Trailing Edge Channel

2013 ◽  
Author(s):  
Rui Kan ◽  
Li Yang ◽  
Jing Ren ◽  
Hongde Jiang
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Local Heat Transfer ◽  
Large Aspect Ratio ◽  
Thermochromic Liquid Crystal ◽  
Transfer Coefficients ◽  
Trailing Edge ◽  
Average Heat ◽  
Edge Channel ◽  
Heat Transfer Coefficients

Thermal issues of gas turbine blade trailing edge are complex and challenging, due to limited space for effective cooling. The trailing edge cavities are usually large aspect ratio ducts with lateral ejection. The objective of this study is to investigate the effects of different rib configurations and lateral ejection on heat transfer characteristics inside a trailing edge channel. The investigations were conducted on a large aspect ratio wedge-shaped channel with square ribs of e/Dh = 0.05, under Reynolds number 15,000. Twelve different configurations were tested: 1)three rib types, including a symmetry V-shaped rib configuration and two non-symmetry V-shaped rib configurations, of which the rib angles are 60 degrees, 2) two rib pitches, P/e = 10 and P/e = 5, 3) two flow directions, with an open tip outlet or with lateral ejection. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal experimental method. The configurations were also investigated numerically for the detailed flow field and for the validation of CFD codes. Results show that with lateral ejection, the heat transfer coefficients decrease from inlet to outlet. The heat transfer near the ejection holes is enhanced while heat transfer coefficients near the wall opposite to the exit holes decrease. The curvature of the streamlines creates a large separation area near the end of the channel and thus results in low local heat transfer coefficients. The P/e = 10 configurations have higher average heat transfer compared with P/e = 5 configurations. Average heat transfer coefficient is the highest with the center of the V-shaped rib placed at the middle of the channel, and is the lowest when the V-shaped rib center is located near the narrow part of the channel.

Visualization of Convective Boiling Heat Transfer in Single Micro-Conduits With Different Shapes of Cross Sections

2005 ◽  
Author(s):  
Tzu-Hsiang Yen ◽  
Masahiro Shoji ◽  
Fumio Takemura ◽  
Yuji Suzuki ◽  
Nobuhide Kasagi
Keyword(s):  
Heat Transfer ◽  
Flow Pattern ◽  
Cross Sections ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Flow Patterns ◽  
Transfer Coefficients ◽  
Convective Boiling ◽  
Heat Transfer Coefficients ◽  
Square Microchannel

Visualization experiments of convective boiling in transparent single micro conduits with the same hydraulic diameter but different cross sections are carried out with simultaneous measurement of local heat transfer coefficients and pressure losses. Two different cross sections with the same similar hydraulic diameters are applied: A circular microtube of 210μm in diameter and a square microchannel of 214μm × 214μm cross section. ITO/Ag thin film of 100 nm is sputtered on the outer surface of the conduits for the direct joule heating. The convective boiling shows some periodic variation of different flow patterns in both square and circular conduits. These flow patterns include bubbly, plug, slug, annular and capillary flows. The capillary flow pattern is the independent liquid droplets moving in the flow direction and very rarely observed in conventional tubes. The reason of such variation of flow patterns is that confined spaces limit the bubble growth in radial direction. So the nucleation bubble grows in both upstream and downstream and makes the flow pattern varies radically. The square microchannel conduit has more simple flow pattern variation, more nucleation bubbles and larger local heat transfer coefficients at lower vapor quality. It is due to that corners of the square microchannel act as helps nucleation cavities. Corners also promotes the formation of liquid film and the contact line between liquid and wall, which can stabilize the flow field. Local heat transfer coefficients decrease with increasing local vapor qualities. Local heat transfer coefficients increase with increasing boiling number but have their maximum value when boiling number reaches critical value. Such peculiar heat transfer characteristics can also be explained by the visualization results.

Design of an axially concave pad profile for a large turbine tilting-pad bearing

2016 ◽  
Vol 231 (4) ◽  
pp. 479-488 ◽  
Author(s):  
Sebastian Kukla ◽  
Nico Buchhorn ◽  
Beate Bender
Keyword(s):  
Heat Transfer ◽  
Film Thickness ◽  
Load Capacity ◽  
Measurement Data ◽  
Tangential Direction ◽  
Measured Temperature ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Tilting Pad ◽  
Calculation Results

To improve operational safety and/or achieve a higher load capacity of turbine tilting-pad bearings, an axially concave pad profile is presented. The thermal and mechanical stress of the loaded pads of a test bearing in load between pivot configuration has been analysed. Both film thickness and pressure distribution have been measured at a very high resolution. A fluid film calculation program in combination with a finite-volume-based structural mechanics program is used to simulate the deformation of a single pad under high circumferential speeds. In this context, the axial and tangential heat transfer coefficients of the pad surface, which act as boundary conditions for the calculation of the 3D temperature distribution, are determined using an optimization process. Herein, the match of predicted and measured pad temperatures is the goal. It can be shown that there must be a huge difference in heat transfer in axial and tangential direction in order to match the large measured temperature gradient in circumferential direction. Based on the measured deformed profile the program code is used to derive a concave pad profile, which will result in an axially non-arched sliding surface under the expected thermal load. Therefore, an iterative simulation procedure is used. By decreasing the axial arching of the pad and thus the large film thickness at the axial ends using an improved profile designed for a specific operation point, the minimum film thickness and maximum pad temperature can be influenced beneficially. The comparison of measurement data and calculation results shows very good agreement regarding the pad deformations. The results indicate that by axially concave profiling of the loaded pads of a large tilting-pad bearing for a specific operation point, the static characteristics in the form of temperature, film thickness and load capacity can be improved.

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