Effectiveness-NTU Relationship for a Counterflow Heat Exchanger Subjected to an External Heat Transfer

2004 ◽  
Vol 127 (9) ◽  
pp. 1071-1073 ◽  
Author(s):  
Gregory F. Nellis ◽  
John M. Pfotenhauer
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Exchanger ◽  
Numerical Model ◽  
Analytical Solution ◽  
Convenient Method ◽  
Process Engineering ◽  
External Heat ◽  
External Heat Transfer ◽  
External Heat Flux

This paper presents the analytical solution for the effectiveness of a counterflow heat exchanger subjected to a uniformly distributed, external heat flux. The solution is verified against conventional ε-NTU relations in the limit of zero external heat flux. This situation is of interest in applications such as cryogenic and process engineering, and the analytical solution provides a convenient method for treating differential elements of a heat exchanger in a numerical model.

Effectiveness of Counter Flow Microchannel Heat Exchangers Subjected to External Heat Transfer and Internal Heat Generation

2009 ◽  
Author(s):  
B. Mathew ◽  
T. J. John ◽  
H. Hegab
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Ambient Temperature ◽  
Heat Generation ◽  
Internal Heat ◽  
External Heat ◽  
Internal Heat Generation ◽  
Counter Flow ◽  
External Heat Transfer ◽  
The Individual

The effect of external heat transfer and internal heat generation on the thermal performance of a balanced counter flow microchannel heat exchanger is theoretically analyzed in this paper. External heat transfer occurs due to the thermal interaction between ambient and the fluids. Internal heat generation takes into account the heat generated inside the channels due to the conversion of pumping power into heat. One-dimensional governing equations for both fluids were developed and solved to obtain the axial temperatures. The governing equations were solved using a 2nd order finite difference scheme. The effectiveness of the fluids is dependent on NTU, the ambient temperature, the thermal resistance between the individual fluids and the ambient and the pumping power. With increase in ambient temperature the effectiveness of the hot and cold fluid decreased and improved, respectively. On the other hand, reductions in the ambient temperature always lead to the improvement and degradation of the hot and cold fluid effectiveness, respectively. Depending on the ambient temperature, the thermal resistance between the individual fluids and the ambient increased or decreased the effectiveness of the fluids. Internal heat generation always reduced and improved the hot and cold fluid effectiveness, respectively. The combined effect of external heat transfer and internal heat generation on the effectiveness of the fluids depends on the net amount of heat gained/lost by the individual fluids. The effectiveness of a microchannel counter flow heat exchanger is found to be better than of a parallel flow heat exchanger subjected to the same set of external conditions. The model developed in this paper has been verified using existing models that consider each of these effects individually.

Evaluation of the External Heat Transfer Coefficients in the High-Pressure Turbine of a Full-Scale Core Engine

1996 ◽  
Author(s):  
A. R. Narcus ◽  
H. R. Przirembel ◽  
F. O. Soechting
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
High Pressure ◽  
Data Reduction ◽  
Turbine Blades ◽  
External Heat ◽  
Transfer Coefficients ◽  
High Pressure Turbine ◽  
External Heat Transfer ◽  
Heat Transfer Coefficients

The external heat transfer coefficients, necessary for efficient and accurate turbine blade design, have been quantified using three independent methods of data reduction for the high-pressure turbine blades tested in a core engine. Two of the methods utilized external and internal thermocouple data to determine the heat transfer coefficient levels while the third method required the applied heat-flux levels to determine the coefficients. The heat-flux was calculated from the measured potential difference between thermocouple pairs embedded in the external and internal walls of the turbine blades. The instrumented airfoils were calibrated in a laboratory prior to engine testing. The results of the experimental test showed external heat transfer coefficients could be obtained in an engine environment with a ±3.2% minimum absolute uncertainty. All three data reduction methods produced external heat transfer coefficients within a high degree of accuracy and precision for all data locations on the instrumented airfoils. The three data reduction approaches are presented as well as the data for a specific location on a turbine blade for each method of data reduction. In addition, pre-test calibration procedures and data are discussed along with supporting engine instrumentation used to verify the data acquired during the experimental evaluation.

Performance Evaluation of Parallel Flow Microchannel Heat Exchanger Subjected to External Heat Transfer

2007 ◽  
Author(s):  
B. Mathew ◽  
H. Hegab
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
External Heat ◽  
Parallel Flow ◽  
External Heat Transfer ◽  
Microchannel Heat Exchanger ◽  
External Heating ◽  
Wide Range ◽  
The Individual

In this paper the effect of constant external heat transfer on the performance of a two fluid balanced parallel flow microchannel heat exchanger is analyzed. A mathematical model is developed for predicting the effectiveness-NTU relationship of both the fluids. Theoretical analysis is presented for various cases of external heating over a wide range of NTU. External heating improved the effectiveness of the cold fluid but degraded the effectiveness of the hot fluid while external cooling produced the opposite changes in the effectiveness of the coolants. The extent of improvement or degradation depended on the level of external heating/cooling. A term referred to as performance factor is used to assess the degree of improvement or degradation of the effectiveness of the individual fluids. Experiments conducted on two microchannel heat exchangers by subjecting the coolants to 5% and 10% external heating have been presented in this paper. The experimental value of NTU varied from 0.42 to 1.76. Good agreement is observed between the theoretical predictions and experimental results. The results presented in this paper are nondimensional; thus they can be utilized irrespective of the dimensions of parallel flow microchannel heat exchangers as well as the type of coolant.

scholarly journals Application of a Heat Flux/Calorimeter-Based Method to Assess the Effect of Turbulence on Turbine Airfoil Heat Transfer

1994 ◽  
Author(s):  
B. Glazer ◽  
H. K. Moon ◽  
L. Zhang ◽  
C. Camci
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Cooling System ◽  
External Heat ◽  
Experimental Methodology ◽  
Local Cooling ◽  
External Heat Transfer ◽  
Free Stream Turbulence

The accurate prediction of turbine airfoil metal temperatures remains one of the critical issues in the development of high efficiency engines. Free-stream and wake-generated turbulence plays a major role in the external heat transfer of the cooled airfoils. Turbulence simulation experimental methodology has been employed to provide external heat load similarity between the engine and the elevated temperature cascade rig conditions. The methodology is based on simulation of turbulence intensity to produce equal mainstream heat transfer effects at the stagnation region of the airfoil in both engine and cascade environments. A recently completed fill-scale hot cascade facility provides a realistic simulation of an actual engine in terms of gas-side and coolant-side heat transfer. Significant attention is paid to emulating the free-stream turbulence environment of an actual engine. Indirect measurements of free-stream turbulence are performed with a custom designed calorimetric probe and heat flux probe. Well established stagnation point heat transfer correlations are used to deduce the free-stream turbulence intensity. The cascade rig provides a detailed map of local cooling effectiveness along the airfoil, which can be controlled by varying gas-side and coolant-side convective heat transfer. Results of the experimental study demonstrate the practical benefits of this methodology for more accurate evaluation of the airfoil external heat transfer, particularly when a combustor system is redefined or an engine is uprated and the airfoil cooling system has to be modified.

scholarly journals Pool Boiling of Nanofluids on Biphilic Surfaces: An Experimental and Numerical Study

Nanomaterials ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 125
Author(s):  
Eduardo Freitas ◽  
Pedro Pontes ◽  
Ricardo Cautela ◽  
Vaibhav Bahadur ◽  
João Miranda ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Numerical Model ◽  
Heat Dissipation ◽  
Numerical Study ◽  
Pool Boiling ◽  
Temperature Drop ◽  
Surface Cooling ◽  
Average Temperature ◽  
Gold Silver

This study addresses the combination of customized surface modification with the use of nanofluids, to infer on its potential to enhance pool-boiling heat transfer. Hydrophilic surfaces patterned with superhydrophobic regions were developed and used as surface interfaces with different nanofluids (water with gold, silver, aluminum and alumina nanoparticles), in order to evaluate the effect of the nature and concentration of the nanoparticles in bubble dynamics and consequently in heat transfer processes. The main qualitative and quantitative analysis was based on extensive post-processing of synchronized high-speed and thermographic images. To study the nucleation of a single bubble in pool boiling condition, a numerical model was also implemented. The results show an evident benefit of using biphilic patterns with well-established distances between the superhydrophobic regions. This can be observed in the resulting plot of the dissipated heat flux for a biphilic pattern with seven superhydrophobic spots, δ = 1/d and an imposed heat flux of 2132 w/m2. In this case, the dissipated heat flux is almost constant (except in the instant t* ≈ 0.9 when it reaches a peak of 2400 W/m2), whilst when using only a single superhydrophobic spot, where the heat flux dissipation reaches the maximum shortly after the detachment of the bubble, dropping continuously until a new necking phase starts. The biphilic patterns also allow a controlled bubble coalescence, which promotes fluid convection at the hydrophilic spacing between the superhydrophobic regions, which clearly contributes to cool down the surface. This effect is noticeable in the case of employing the Ag 1 wt% nanofluid, with an imposed heat flux of 2132 W/m2, where the coalescence of the drops promotes a surface cooling, identified by a temperature drop of 0.7 °C in the hydrophilic areas. Those areas have an average temperature of 101.8 °C, whilst the average temperature of the superhydrophobic spots at coalescence time is of 102.9 °C. For low concentrations as the ones used in this work, the effect of the nanofluids was observed to play a minor role. This can be observed on the slight discrepancy of the heat dissipation decay that occurred in the necking stage of the bubbles for nanofluids with the same kind of nanoparticles and different concentration. For the Au 0.1 wt% nanofluid, a heat dissipation decay of 350 W/m2 was reported, whilst for the Au 0.5 wt% nanofluid, the same decay was only of 280 W/m2. The results of the numerical model concerning velocity fields indicated a sudden acceleration at the bubble detachment, as can be qualitatively analyzed in the thermographic images obtained in this work. Additionally, the temperature fields of the analyzed region present the same tendency as the experimental results.

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