Internal Heat Transfer Coefficient Research Articles

Comparative analysis and experimental evaluation of single slope solar still augmented with permanent magnets and conventional solar still

Experimental and theoretical evaluation of conventional solar still (CSS) and CSS augmented with permanent ferrite ring magnets (MSS) have been reported. Mathematical model proposed by Kumar & Tiwari has been used for the evaluation of internal heat transfer coefficients, internal efficiency, exergy efficiency, and exergy destruction. The average partial pressure difference between basin water and inner condensing cover surface for MSS has been found to be 77.99% higher than CSS when experiments were carried out for 13 h. The maximum value of evaporative heat transfer coefficient of MSS leads over the CSS by 28.65% at 13:00 h. The distillate yield recorded during the experimentation for MSS is 49.22% higher as compared to CSS. It seems that the magnetization of water has enhanced the overall internal efficiency and exergy efficiency of MSS over CSS by 49.17% and 110.26% respectively. In MSS the presence of permanent magnets has remarkably reduced the exergy destruction in the basin area. Augmentation of magnets with the CSS has significantly enhanced the distillate output of MSS. It has been found that the theoretical results obtained from Kumar & Tiwari numerical model are in good agreement with the experimental results obtained from CSS and MSS.

New methods for numerical estimation of convective heat transfer coefficient in circular ducts

Estimating the internal heat transfer coefficient in ducts is becoming increasingly important in many engineering applications. Unlike traditional estimation methods which seek to recover the unknown coefficient by using optimization tools such as iterative methods or linear system solvers, new solution methods based on forward/inverse analytical solutions have gained noticeable interest. Two recently proposed non traditional methods are an explicit Fourier analysis-based (EFA) reconstruction method of Bazán and Bedin and a filtered reciprocity functional (FRF) approach of Mocerino et al. Both methods provide approximate solutions that are expressed as a sum of weighted harmonics where the number of terms used in the reconstruction depends on the noise level in the data and is selected using the Morozov's discrepancy principle (DP). In the present work, these methods are analyzed to identify similarities and differences as well as to introduce improvements. In particular, limitations of the FRF method in determining the truncation parameter are discussed and circumvented. This gives rise to new methods for heat flux estimation that are easy to implement and extremely inexpensive when compared to existing techniques. To illustrate the effectiveness of the new methods we present numerical results on several test problems including an analytical model obtained by synthesizing experimental data reported by Bozzoli et al. and several benchmark problems reported by Mocerino et al.

Influence of ICHTC correlations on the thermal characterization of façades using the quantitative internal infrared thermography method

The thermal characterization of façades of the existing building stock is essential to establish optimal energy conservation measures. There are different methods to characterize thermal properties of façades. The quantitative internal infrared thermography method is among those most developed. Given the existing differences in the scientific literature among the proposals of the method, this study analysed the influence of the internal convective heat transfer coefficient (ICHTC). In total, 25 correlations of temperature differences (temperature of the wall and internal air temperature) were analysed, as well as 20 correlations of dimensionless numbers. To do this, an experimental campaign was performed in 3 façades belonging to the most representative building periods of the building stock in Spain. First, a cluster analysis was carried out to determine similarities among the equations analysed, using the Ward method as an agglomerative hierarchical method and the Euclidean distance as an association measurement. In total, 12 and 8 groups were obtained for correlations of temperature difference and of dimensionless numbers, respectively. Afterwards, results associated with each approach were obtained. These results showed that a better adjustment was obtained for correlations of dimensionless numbers by using the approach of convection and radiation, with an average value of representative results higher than 80%.

A Three-Dimensional Conjugate Approach for Analyzing a Double-Walled Effusion-Cooled Turbine Blade

A double-wall cooling scheme combined with effusion cooling offers a practical approximation to transpiration cooling which in turn presents the potential for very high cooling effectiveness. The use of the conventional conjugate computational fluid dynamics (CFD) for the double-wall blade can be computationally expensive and this approach is therefore less than ideal in cases where only the preliminary results are required. This paper presents a computationally efficient numerical approach for analyzing a double-wall effusion cooled gas turbine blade. An existing correlation from the literature was modified and used to represent the two-dimensional distribution of film cooling effectiveness. The internal heat transfer coefficient was calculated from a validated conjugate analysis of a wall element representing an element of the aerofoil wall and the conduction through the blade solved using a finite element code in ANSYS. The numerical procedure developed has permitted a rapid evaluation of the critical parameters including film cooling effectiveness, blade temperature distribution (and hence metal effectiveness), as well as coolant mass flow consumption. Good agreement was found between the results from this study and that from literature. This paper shows that a straightforward numerical approach that combines an existing correlation for film cooling from the literature with a conjugate analysis of a small wall element can be used to quickly predict the blade temperature distribution and other crucial blade performance parameters.

Experimental investigation on heat dissipation of partitioned and non-partitioned rectangular enclosures under no-wind condition

Under no-wind condition, heat dissipation in partitioned and non-partitioned rectangular enclosures formed by quartz glass cover and base plate made of composite stone was experimentally investigated. Internal and external combined heat transfer coefficients hin and hex of enclosure, total thermal resistance Rtot of heat dissipation and correlations of average radiative and convective heat loss Nusselt numbers outside the glass cover were obtained and discussed. Also, the impacts of the gradient parameter k and magnitude of heat flux and tilt angle were compared. Results show that the local temperature differences (tl − ta) on the base plate and glass cover change in the same trend, while the impacts of partitions on average temperature on glass cover is greater than that on base plate. Compared with non-partitioned condition, hex is decreased and hin is increased, meanwhile, Rtot is higher when tilt angle θ < 60° for partitioned condition. Rtot for non-uniform heat flux boundary condition is higher than that for uniform heat flux boundary condition. Both for partitioned and non-partitioned conditions, thermal resistance Rex outside the glass cover is the main limited factor to control entire heat transfer process under θ < 60°. For non-partitioned condition, average convective heat loss Nusselt number Nucm increases with the augment of tilt angle, while for partitioned enclosure, Nucm for θ = 60° shows no relative tendency when k = 0 and k = 0.1. As for Nurm, the influence of tilt angle is less pronounced when compared with other parameters.

Experimental investigation of condensation flows in circular cross-section channel

In this work, condensation flows of pure ethanol in a horizontal water-cooled circular cross-section heat exchanger were investigated for a mass velocity ranging between 30 and 100 kg m−2 s−1. The inner channel of condensation has inner and outer diameters of 4 mm and 10 mm, respectively, with a heat exchange length of 1020 mm. An adiabatic section consisted of a transparent circular cross-section channel, of 4 mm inner diameter and of 130 mm visible length, was placed downstream of the condenser in order to visualise the condensation flows by shadowgraph technique. Two main flow regimes were identified: the wavy-stratified regime and the wavy-annular regime. A flow regime map was developed and compared with flow regime maps available in the literature. Besides, the local vapour quality and internal heat transfer coefficient were determined and compared with prediction models of the literature. Finally, the total pressure drop in the condenser was determined.

Experimental investigation of circumferentially non-uniform heat flux on the heat transfer coefficient in a smooth horizontal tube with buoyancy driven secondary flow

In this experimental investigation the influence of non-uniform heat flux distributions on the internal heat transfer coefficient in a horizontal circular tube was studied for liquid water. The tube had an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions were studied for Reynolds numbers ranging from 650 to 2600 at a Prandtl number of approximately 6.5. Heat flux distributions included fully uniform heating (which had a circumferential angle span of 360°) and different partial uniform heat flux distributions with angle spans of 180° or 90° at different circumferential positions. Depending on the angle span, local heat flux intensities ranging from 1658 W/m2 to 6631 W/m2 were tested. Results indicate that the average steady state Nusselt number is greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for cases where the applied heat flux was positioned on the lower half (in terms of gravity) of the tube circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Smaller angle spans produced lower heat transfer coefficients. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for angle spans of 180° and 90° which correlated 92% and 96% of the data respectively within 3% of the measured Nusselt number.

State-of-the-Art Cooling Technology for a Turbine Rotor Blade

Effective internal and external cooling of airfoils is key to maintaining component life for efficient gas turbines. Cooling designs have spanned the range from simple internal convective channels to more advanced double-walls with shaped film-cooling holes. This paper describes the development of an internal and external cooling concept for a state-of-the-art cooled turbine blade. These cooling concepts are based on a review of literature and patents, as well as, interactions with academic and industry turbine cooling experts. The cooling configuration selected and described in this paper is referred to as the “baseline” design, since this design will simultaneously be tested with other more advanced blade cooling designs in a rotating turbine test facility using a “rainbow turbine wheel” configuration. For the baseline design, the leading edge is cooled by internal jet impingement and showerhead film cooling. The midchord region of the blade contains a three-pass serpentine passage with internal discrete V-shaped trip strips to enhance the internal heat transfer coefficient (HTC). The film cooling along the midchord of the blade uses multiple rows of shaped diffusion holes. The trailing edge is internally cooled using jet impingement and externally film cooled through partitioned cuts on the pressure side of the blade.

Filtered reciprocity functional approach to estimate internal heat transfer coefficients in 2D cylindrical domains using infrared thermography

Performance estimation of heat exchanger apparatuses requires a proper knowledge of the thermo-fluid dynamic interaction between fluid and device. One of the aspects to take into account is related to the estimation of the internal convective heat transfer coefficient. This requires the knowledge of the internal temperature and the internal wall heat flux, although these quantities are complicated to be measured. Therefore, the use of inverse problem techniques may be very useful for estimating these quantities. This approach deals with the estimation of the local internal properties, given some external temperature measurements, possibly by means of contactless experimental methodologies (i.e. infrared camera imaging).The solution strategy here presented, for a 2D model, is based on the Reciprocity Functional approach (RF), which requires the solution of two auxiliary problems that are solved, in this paper, by means of the Classical Integral Transform Technique (CITT). The RF approach coupled with CITT, presents some advantages over other techniques since its solution is completely analytical and therefore computationally fast, and it does not require any numerical simulation. Moreover, it deals with the noise content of the experimental data, filtering it and overcoming the documented limits of the classical RF approach. This original method, named Filtered Reciprocity Functional (FRF) approach, was applied to different test cases using simulated measurements, and the results were compared to both the exact and the reconstruct solutions obtained by the Truncated Singular Value Decomposition (TSVD) method, showing a good agreement. Finally, the FRF approach was applied to an experimental case in order to test its robustness when dealing with real data.

Thermo-hydraulic characteristics and second-law analysis of a single-phase natural circulation loop with end heat exchangers

In this work, a three-dimensional transient numerical simulation of heat transfer characteristics in a single-phase natural circulation loop (SPNCL) with water as the working fluid in the loop and end heat exchangers is performed. A 3-D computational model of a SPNCL with end heat exchangers is developed. The second-law analysis for the SPNCL is presented and total entropy generation is given. Meanwhile, the transient response is obtained based on the finite difference method solving the one-dimensional momentum and energy equations. Firstly, a mesh dependence test and a model verification with the laminar correlation presented by Vijayan are conducted. Then the effects of driving temperature difference and heating or cooling fluid velocity on the thermos-hydraulic performance and dimensionless entropy generation number are investigated by fixing the cooling fluids temperature of 273K. Results show that a stable flow can be reached for all the cases. The formation of the stable natural circulation is the consequence of coupling between heating and cooling fluid. The time of quiescent state is relatively short due to the thermal asymmetry of the SPNCL. The mass flow rate and heat transfer rate at steady state increase with the increase of driving temperature difference and heating or cooling fluid velocity. It is obviously shown the internal heat transfer coefficient is mainly dependent on the driving temperature difference and increases with the increase of driving temperature difference. The thermal effectiveness and dimensionless entropy generation number also increase with the increase of driving temperature difference and heat transfer irreversibility is dominated in total irreversibility. Moreover, the time evolution of dimensionless mass flow rate shows a damped oscillation behavior.

Energetic and Exergetic Performance of a Solar Flat-Plate Collector Working With Cu Nanofluid

The evaluation of the performance and characteristics of a solar flat-plate collector (FPC) are reported for domestic and industrial requirements in the existing literature. A computer code was developed using matlab to model and evaluate the energetic and exergetic performance of a nanofluid-based FPC for steady-state and laminar conditions. The analysis was performed using practical geometry data, especially the absorber emittance, for a standard collector. Linear pressure losses in manifolds were taken into account, and a more accurate exergy factor corresponding to a correct value of 5770 K for the sun temperature was employed. The results demonstrate that copper–water nanofluid has the potential to augment the internal convection heat transfer coefficient by 76.5%, and to enhance the energetic efficiency of the collector from 70.3% to 72.1% at 4% volume concentration, when compared to the values with water. Additionally, it was revealed that copper nanofluid is capable of increasing the collector fluid's outlet temperature and decreasing the absorber plate's mean temperature by 3 K. The addition of nanoparticles to the water demonstrated a reduction in the total entropy generation by the solar FPC. Furthermore, increasing the nanoparticle size reflected a reduction in the overall performance of the solar collector.

Internal heat transfer coefficient estimation in three-dimensional ducts through the reciprocity functional approach – An analytical approach and validation with experimental data

In the current paper it is presented and tested an innovative method for estimating internal heat transfer coefficients in ducts adopting only thermal measurements acquired on the external wall surface. This approach is based on the reciprocity functional analysis, which is a powerful non-intrusive inverse problem technique. The adoption of this technique is promising since it avoids intrusive measurements, it is fully non-iterative and the computational time and cost are very limited. In this paper, it is presented an extension of the classical reciprocity functional methodology in the sense that a fully analytical expression, obtained by the integral transform technique, is developed for estimating the internal heat transfer coefficient in a three-dimensional problem. Such methodology, avoiding the solution of linear systems, reduces the computational costs that are massive when the traditional approaches are applied to three dimensional problems. The proposed procedure is first validated adopting synthetic temperature data and then tested using real temperature measurements acquired by an infrared camera. The results highlight that the methodology is able to recover the unknown functions in a very short computational time with a good accuracy.

Experimental study on performance of passive and active solar stills in Indian coastal climatic condition

This present work is aimed to examine the effect of mass flow rate on distillate output and performance of a solar still in active mode. Outdoor experiments were conducted at the coastal town, Kakinada (16°93′N/83°33′E), Andhra Pradesh, India. A solar still with a 30° of fixed cover inclination, 1 m2 of effective basin area, and a flat-plate collector (FPC) with an effective area of 2 m2 were used. An attempt was also made earlier in passive mode to optimize the water depth for the same solar still for maximum yield and distillation efficiency. For the passive still, it is observed that the capacity of heat storage and heat drop are significant parameters that affect the still performance. For the selected still design, the study reveals that 0.04 m water depth is the optimum value for specific climatic conditions. In the active solar still, with the optimum water depth, different flow rates of 0.5, 1 and 1.5 L/min are considered through FPC. It is observed that both the mass flow rate and the variation of internal heat transfer coefficients with the mass flow rate have a significant effect on the yield and performance of the still. The experimental results show that the combination of 1.5 L/min mass flow rate and an optimum water depth of 0.04 m leads to a maximum yield for the active solar still. The enhanced yield of the active solar still is 57.55%, compared with that of the passive solar still, due to increase in area of radiation collection and more heat absorption rate.

Asymmetrical Non-Uniform Heat Flux Distributions For Laminar Flow Heat Transfer With Mixed Convection In a Horizontal Circular Tube

ABSTRACTNon-symmetric heat flux distributions in terms of gravity in solar collector tubes influence buoyancy-driven secondary flow which has an impact on the associated heat transfer and pressure drop performance. In this study the influence of the asymmetry angle (0°, 20°, 30° and 40°) with regard to gravity for non-uniform heat flux boundaries in a horizontal circular tube was investigated numerically. A stainless steel tube with an inner diameter of 62.68 mm, a wall thickness of 5.16 mm, and a length of 10 m was considered for water inlet temperatures ranging from 290 K to 360 K and inlet Reynolds numbers ranging from 130 to 2000. Conjugate heat transfer was modelled for different sinusoidal type outer surface heat flux distributions with a base-level incident heat flux intensity of 7.1 kW/m2. It was found that average internal heat transfer coefficients increased with the circumferential span of the heat flux distribution. Average internal and axial local heat transfer coefficients and overall friction factors were at their highest for symmetrical heat flux cases (gravity at 0º) and lower for asymmetric cases. The internal heat transfer coefficients also increased with the inlet fluid temperature and decreased with an increase in the external heat loss transfer coefficient. Friction factors decreased with an increase in fluid inlet temperature or an increase in the external heat loss transfer coefficients of the tube model.

Controlling the Internal Heat Transfer Coefficient by the Characteristics of External Flows

The engineering-physical fundamentals of substance synthesis in a boiling apparatus are presented. We have modeled a system of automatic stabilization of the maximum internal heat transfer coefficient in such an apparatus by the characteristics of external flows on the basis of adaptive seeking algorithms. The results of operation of the system in the shop are presented.

Choice of Efficient Linear Scale and Generalized Description of Internal Heat Transfer Coefficient in Porous Structures

The expedience of using the ratio of inertial β and viscous α hydraulic coefficients of a fluid flow in porous structures as the characteristic linear scale, when generalizing the experimental data on internal heat transfer in porous media, is shown. It is demonstrated that the correlation Nu = A · Pe, with both criteria based on β/α ratio, most efficiently describes the experimental data for a wide set of ordered and disordered porous structures, including sintered spheres, network materials, sintered felt and cellular foams of high porosity. The coefficient A depends on porosity and is equal to 0.004 for spheres, networks and felts, and 0.0004 for foams. For any specific case the values of α and β coefficients can be readily obtained from testing materials under consideration, control samples, or full-scale articles.

Experimental thermal performance study of natural graphite as sensible heat storage material using different heat transfer fluids

ABSTRACTIn this paper thermal performance of graphite-based sensible heat storage system with embedded helical coil in rectangular shell was studied. Plain water at four flow rates (0.25 LPM–1.0 LPM) and four inlet temperatures (60°C–90°C) was passed through the graphite bed and charging time was measured. Expanded graphite/water suspension and Al2O3/water nanofluid were also used to study charging behavior of graphite. Results showed that charging time of packed bed was reduced with increase in flow rate and inlet temperature of heat transfer fluid. Charging time using expanded graphite/water solution and nanofluid was 14.2% and 21.2% lesser than water.Abbreviations: hi: internal heat transfer coefficient (W m−2 K−1); HTF: Heat transfer fluid; ho: External heat transfer coefficient (W m−2 K−1); LPM: liter per minute; k: thermal conductivity (W m−1 K−1); TSU: Thermal storage unit

Identification of heat transfer coefficient through linearization: explicit solution and approximation

We derive an explicit reconstruction formula for the heat flux on the internal surface of a duct based on external temperature data. With the heat flux at hand we determine the internal convective heat transfer coefficient. Technically, by determining explicitly the singular system of a compact operator and assuming noise-free data, the heat flux is expressed as an infinite singular value expansion, whereas for noisy data, the truncated expansion is used as regularized solution. An error bound is derived for the case where the truncation parameter is determined by the Morozov’s discrepancy principle. The method has high flexibility in that it is readily adapted to the finite data case without any substantial modification. Moreover, it does not require any linear system solver and it is therefore easy to implement and extremely inexpensive when compared to existing heat transfer coefficient estimation techniques. The method is illustrated by means of numerical results using both synthetic and experimental data.

Effect of flat plate collectors in parallel on the performance of the active solar still for Indian coastal climatic conditions

ABSTRACTThe effect of flat-plate collectors (FPCs) when connected in parallel on, in particular, the heat transfer coefficients and, in general, distillate output and the efficiency of solar still has been studied in the active mode. The solar still has an effective basin area of 1 m2 and fixed cover inclination of 30°. Each FPC with an effective area of 2 m2 is attached to the solar still. An effective collector inclination of 5° is used. Experiments have been conducted for 24 h in summer for the active solar distillation system at Kakinada (16°.93′N/83°.33′E), India, which has coastal climatic conditions. The aim of the present work is to study the variation in internal heat transfer coefficients with respect to the number of FPCs coupled with the solar still in parallel. It is understood that the heat transfer coefficients depend significantly on the size of the collector. The size of the collector is increased by increasing the number of collectors. It is also observed that this leads to an increase in distillate output but a decrease in efficiency of the solar still.

Thermal Modeling of Indirect Solar Drying System: An Experimental Validation

The expressions for crop and moist air temperatures, drying rate and efficiency of indirect solar drying with phase change material (PCM) storage systems in quasi-steady state conditions have been derived. The analysis is based on the basic energy balance for the system. A computer model has been developed to predict the performance of the solar dry-ers. Experimental validation of the thermal model has been carried out by using modified heat transfer coefficients. Internal heat and mass transfer coefficients have been evaluated with PCM for March 24, 2014 in Varanasi, India. A fair agreement has been observed between theoretical and experimental results by using the modified internal heat and mass transfer coefficients.