Contribution Of Heat Transfer Research Articles

Optimization of heat transfer in a grooved pipe model by Stochastic Algorithms and DOE based RSM

Additive manufacturing is a promising candidate to eliminate design constraints in thermal-fluid engineering problems as it enables optimized free-form designs to be produced. The brute force optimization needs too many experiments and/or analyses to obtain an optimized design. However, the same engineering design can be achieved by fewer experiments by using design of experiment (DOE) methodology, which minimizes the work load. In this study, local and mean heat transfer coefficients for a grooved pipe model are optimized and the system constraints are determined in compatible with the additive manufacturing method. DOE based response surface methodology (RSM) and an in-house developed optimization code are used to obtain the optimal local and mean heat transfer coefficients. The analyses are performed for both laminar and turbulent flow cases for understanding the backward and forward effects of grove dimensions better. The comparison of the recirculation regions in the groove geometries for optimal cases of laminar and turbulent flows reveals that the grooves in the pipe do not have to be uniform along the streamwise direction to obtain optimal heat transfer for local needs. This optimization approach provides both local and global controllability of heat transfer and the heat transfer values can be increased up to 20% in this thermal-fluid engineering problem. In addition, pressure drop and heat transfer contribution ratio of each grooved section are presented in detail.

Radiative heat transfer in FLiBe molten salt participating medium in a vertical heated tube under forced and mixed convection laminar flows

The contribution of radiative heat transfer (RHT) to convective heat transfer in a heated vertical pipe with laminar flow and constant wall temperature, for FLiBe molten salt, is investigated computationally. The combined effects of conduction in the fluid, forced convection, buoyancy, temperature-dependent physical properties, and thermal radiation are investigated. The P1 approximation is employed in the discretization of the Radiative Transfer Equation (RTE). The COMSOL Multiphysics software is used to generate the numerical solutions. The theoretical analysis developed here demonstrates that only for intermediate values of the optical thickness, i.e. τD~O1, the participating media effects are expected to be important. This analysis is confirmed computationally and a value of τD≈4 is shown to lead to the highest increase in overall heat transfer behavior. Under forced convection, the Nusselt ratio Nutotal/Nuno-rad reaches a peak value of 1.76 at τD=4 and z/D=200, and is less than 1.1 for τD<0.1andτD>60. Under aiding-flow mixed convection, Nutotal/Nuno-rad reaches a peak of 1.34 at τD=4.2 and is less than 1.1 for τD<0.4andτD>36. Under opposing-flow mixed convection, Nutotal/Nuno-rad reaches a peak of 1.81 at τD=4.2 and is less than 1.1 for τD<0.2andτD>50. RHT effects on the overall heat transfer are most pronounced in opposing mixed convection, where Nutotal/Nuno-rad is 2.26 observed at z/D=50. The sensitivity to wall emissivity is evaluated; in forced convection, the peak Nutotal/Nuno-rad is 1.66 at ε = 0 (reflective wall) and 2.04 at ε =1 (absorptive wall). The sensitivity to pipe diameter is also discussed. Overall, for a vertical heated tube, radiative heat transfer effects lead to enhancement of heat transfer by as high as a factor of two, and they depend on the optical thickness of the flow, mixed convection environment (Gr/Re2 and direction of flow relative to the gravitational force), surface emissivity, and entrance effects.

Downhole electric heater with high heating efficiency for oil shale exploitation based on a double-shell structure

To improve the heating efficiency of the downhole electric heaters used in oil shale exploitation, double-shell downhole electric heaters with continuous helical baffles (DS-DEHs) were developed in this study. These heaters reduce heat loss generated by the shell of single-shell downhole electric heaters with continuous helical baffles (SS-DEHs) and reuse the heat loss by driving air flows through outer and inner shell-passes in sequence. Two types of DS-DEHs with three different helical pitches were experimentally studied, and SS-DEHs were set as the control. The results indicated that the effect of mass flow rate on the heating rate is greater than that of heating power. Forced convection is the major heat transfer mode in heater shell-side, and the contribution of shell-side radiant heat transfer mainly depends on the helical pitch, then on the heating power and mass flow rate. Additionally, the heat loss generated by heater shell is mainly dissipated in the form of radiation. The total heat loss of the DS-DEH is 87.16%–96.41% lower than that of the SS-DEH, and its heating efficiency is 1.06–1.17 times than that of SS-DEH, indicating that the double-shell structure can effectively improve the heating efficiency of downhole electric heaters.

Estimating the components of apparent thermal conductivity of soils at various water contents and temperatures

Soil thermal conductivity is an important parameter for estimating soil heat flux and soil thermal regime. Knowledge on the components of apparent soil thermal conductivity (λ) across various water contents (θ) and temperatures is important to accurately understand soil heat transfer mechanisms. In this study, soil thermal conductivity was measured for sandy loam and silty clay soils at various temperatures and air pressures using a transient method. Four components of λ, namely, heat conduction, latent heat transfer by water vapor diffusion, sensible heat transfer by liquid water, and sensible heat transfer by water vapor diffusion were quantified. Results showed that in uniform soils, the magnitudes of sensible heat transfers by liquid water and water vapor were negligible during these transient measurements. The contribution of latent heat transfer through vapor diffusion to total heat transfer increased as temperature increased, and the peak value occurred at an intermediate water content. The water content at which the maximum vapor diffusion occurred varied with soil texture. In addition to the four calculated components, a significant residual contribution to λ caused by an unidentified vapor transfer mechanism was observed between 3.5°C and 81°C. For example, calculations indicated that approximately 66% of the sandy loam λ at θ=0.11 m3 m−3 was caused by an unidentified vapor transfer mechanism at 81°C. This extra contribution by vapor transfer could be explained either as enhanced vapor diffusion or by an advection mechanism. Further investigation is needed to clarify whether enhanced diffusion or advection is occurring in unsaturated soils.

Dynamic wetting and heat transfer during droplet impact on bi-phobic wettability-patterned surfaces

This paper reports the dynamic wetting behavior and heat transfer characteristics for impinging droplets on heated bi-phobic surfaces (superhydrophobic matrix with hydrophobic spots). A non-patterned superhydrophobic and a sticky hydrophobic surface acted as control wettability surfaces. As expected, differences in wetting and heat transfer dynamics were noticeable for all surfaces with the most pronounced variation during the receding phase. During spreading, inertia from the impact dominated the droplet dynamics, and heat transfer was dominated by convection at the contact line and internal flow. As contact line velocities decreased over time, evaporative cooling at the contact line gained importance, especially for the bi-phobic surfaces, where liquid remained trapped on the hydrophobic spots during receding. These satellite droplets increased the contact area and contact line length and assisted heat transfer and substrate cooling after lift-off of the main droplet. Compared with the hydrophobic surface, the contribution of the contact line heat transfer increased by 17%–27% on the bi-phobic surfaces depending on the location of impact relative to the hydrophobic spots. Nonetheless, the bi-phobic surfaces had a lower total thermal energy transfer. However, compared with the plain superhydrophobic surface, heat transfer was enhanced by 33%–46% by patterning the surface. Depending on the application, a trade-off exists between the different surfaces: the sticky hydrophobic surface provides the best cooling efficiency yet is prone to flooding, whereas the superhydrophobic surface repels the liquid but has poor cooling efficiency. The bi-phobic surfaces provide a middle path with reasonable cooling effectiveness and low flooding probability.

Thermal Analysis of a Microhotplate - An Experimental Study

In the present work, experimental and numerical thermal analysis of a microhotplate were performed to evaluate the heat transfer mechanism at small Rayleigh number ( $Ra$ ). The convective and radiative heat transfer coefficient values were evaluated for microhotplate mounted over two different mounting positions (namely model-1 & model-2). At different heat generation ( $Q$ ) values, it was found that the average convective coefficient value was less than $10\frac {W}{m^{2}K}$ , irrespective of the microhotplate mounting position. Comparison of both models at a similar $Q$ ( $\simeq 0.69W$ ) value, reveals that the radiative heat transfer contribution was 33% & 68% for model-1 and model-2 respectively. Based on the results obtained in the present study, a correlation was developed using regression analysis for the convective Nusselt number in terms of Rayleigh number ( $8.84\times 10^{-4}\le \:Ra\:\le 4.94\times 10^{-3}$ ), and the convective Nusselt number obtained from the correlation and present study are within ±17%.

Heat and mass transfer modeling of R-245fa and R1233zd(E) with concurrent boiling and convective evaporation in falling film applications

Abstract Design of some thermal industrial equipment like Heating Ventilation and Air Conditioning falling film evaporators and desalination units requires knowing discrete portion of convective evaporation and boiling. While most models are based on sole dominant evaporation, one small variation in design parameters can increase boiling part and threaten the model. Heat transfer simulation of such equipment is dependent on mass transfer modeling. Therefore, this study uses different mass transfer models and numerically investigates two laminar falling film flows around a horizontal tube heating insomuch that leads to sole evaporation and evaporation with boiling. The multi-phase volume of fluid model is used and the performance of mass transfer models is investigated for heat transfer coefficient, film thickness, bubble generation and liquid superheating. Results show that for 240 mL*min−1 of R-245fa at saturation pressure of 123.8 kPa at wall and saturation temperature difference of below 7 K, main contribution of heat transfer is due to evaporation which is accurately modeled by Tanasawa and Lee phase change models. With increasing wall heat flux, boiling portion signifies. Accordingly, Tanasawa model is not able to simulate boiling part but Lee model predicts total heat transfer coefficient with 7% error. After validating the numerical model, the model has been used for R1233zd(E) as a low Global Warming Potential refrigerant and transition to boiling presence of the falling flow is investigated. Comparison of the results represents governing evaporation up to 7 K of wall and saturation temperature difference of R1233zd(E) at similar saturation temperature of R-245fa.

Wall heat flux partitioning during subcooled flow boiling of NaCl solutions and pure water

A careful evaluation on the wall heat flux partitioning is of great importance for the deep understanding the nucleate flow boiling heat transfer to saline solution. In this work, the portion of each heat transfer contribution was analyzed through wall heat flux partitioning model after model validation. The key parameters used in the model were discussed and the reasonable values were determined. The experimental data of bubble departure diameter, departure frequency, active nucleation site density, bubble waiting time and growth time, were combined into the model to eliminate the uncertainties of empirical closure sub-models. The comparison of heat flux partitioning of subcooled flow boiling between NaCl solution and pure water was presented under different operating conditions. The results showed that for the operating conditions of Qw > 180 kW/m2 or G < 400 kg/(m2·s), the contribution of single-phase liquid convection in the heat transfer of saline solution is greater than that in pure water, due to the bigger bubble diameter and more intensive bubble-induced turbulence. Quenching heat flux for NaCl solution is lower than that in pure water, owing to the smaller bubble departure frequency. For the lower heat flux or higher mass flux conditions, however, the contrary trend was observed. The dominant heat transfer mechanisms for solution and pure water during subcooled nucleate flow boiling were revealed. The interactions between bubble dynamic and heat transfer characteristics were carefully investigated.

Understanding the Contributions of Microscopic Heat Transfer to Thermal Conductivities of Liquid Aldehydes and Ketones by Molecular Dynamics Simulation.

Thermal conductivity measurements for organic molecules are difficult and time-consuming. In this study, the thermal conduction of liquid aldehydes and ketones was simulated using nonequilibrium molecular dynamics, and the thermal conductivity at different temperatures was calculated. The deviation between the calculated values and the experimental data of thermal conductivity is less than 4.86%. By decomposing the heat flux, we found that thermal energy is primarily transferred through the torsion angle, angle bending, Coulomb interaction, and kinetic energy in the liquid state. Moreover, as the molecular chain grows, the thermal energy transmitted through the nonbonded interaction decreases, and the thermal energy transmitted through the intramolecular bonded interaction increases, which indicates a significant relationship between the mechanism of heat transfer and the molecular structure. Using molecular dynamics simulation, this research offers a preliminary understanding of the contributions of microscopic heat transfer to the thermal conductivities of liquid aldehydes and ketones.

Entropy generation for flow boiling on a single semi-circular minichannel

The paper investigates the effects of parameters such as mass flux, heat flux, channel diameter and inlet pressure on the entropy generation in flow boiling inside a semi-circular minichannel. A general entropy generation equation is derived for a single minichannel for flow boiling with developing flow, while also relaxing some heat transfer assumptions such as ΔTTsat≪1. The Romberg integration technique is used to solve for the entropy generation. Our results show that an increase in the mass flux causes an abrupt change in the entropy generation when the flow changes from laminar to transitional flow at small diameters although the effect is less significant at larger diameters. The heat transfer contribution in the entropy generation is also higher than the pressure drop contribution for the larger diameters. The larger channel diameters produce higher entropy generation compared to the smaller diameters for every heat flux investigated due to the increase in heat transfer contribution. An increase in the inlet pressure also decreases the entropy generation for every mass flux and heat flux. The aims of this study are to enhance the knowledge of the effects of heat transfer, pressure drop and flow behavior on the entropy generation, and to encourage researchers and designers to explore more novel features that can take advantage on the minimization of entropy generation.

The Role of Anodization in Naturally Cooled Heat Sinks for Power Electronic Devices

Abstract Effect of anodization on the thermal performance of naturally cooled heat sinks in power electronic devices made of die-cast aluminum alloy A380 and machined aluminum alloy 6061 was investigated experimentally and numerically. Various types of anodization were examined with different thickness of anodic aluminum oxide layer (AAO), pore size distributions, and surface coloring conditions. A customized natural convection and thermal radiation experimental chamber was built to measure the cooling capacity and heat sink temperatures. A 3D numerical model was also developed and validated against the collected data to provide more details into the contribution of the radiation heat transfer. The total emittance of the anodized samples was determined by a Fourier transform infrared reflectometer (FTIR) spectroscopy method. The results show a significant improvement in total hemispherical emissivity from 0.14 to 0.92 in anodized die-cast aluminum samples. This increase resulted in a considerable reduction in overall thermal resistance, up to 15%; where up to 41% of the total heat dissipation was contributed by thermal radiation. In spite of the rather distinguishable surface morphologies, the measurements suggested that thermal emissivity of the anodized die-cast Al A380 and Al alloy 6061 samples were in the same range.

Influence of micropores on structural instability of the combustion wave

The article shows examples of visualization of the process of heat transfer by radiation in unstable combustion modes of porous powder materials, which are in good agreement with the results of high-speed video recording and micropyrometry. The mathematical model and the results of calculating the structure of the combustion wave in the Ni-Al system are presented. The contribution of radiative heat transfer at an adiabatic combustion temperature in the range of 810 % and the effect of its trigger shutdown with decreasing temperature were revealed.

Effects of particle contact characteristics on heat transfer performance of calcined petroleum coke heat exchanger

The present paper investigated the effect of particle contact characteristics (contact area coefficient) on heat transfer performance of heat exchanger. An unsteady heat transfer model of single-layer calcined petroleum coke was established. The average temperature, heat storage, heat transfer contribution rate and wall heat transfer coefficient were studied with different times. The research shows that with increase of contact area coefficient, the heat conduction capacity is enhanced and the average temperature of particles layer reduction rate and the heat storage reduction rate are obviously accelerated. The average contribution rate of solid phase heat transfer, wall comprehensive heat transfer coefficient and heat release rate increase, and the effective heat transfer time decreases. When the contact area coefficient increased from 0.03 to 0.07, the average contribution rate of solid phase heat transfer increased from 83.4% to 90.5%. The wall heat transfer coefficient increased from 12.7 W/(m2·K) to 27.1 W/(m2·K), the heat release rate increased from 83.79% to 98.2%, and the effective heat transfer time decreased from 8.02 h to 5.07 h.

Data-Based RC Dynamic Modelling Incorporating Physical Criteria to Obtain the HLC of In-Use Buildings: Application to a Case Study

This paper reports the application of RC dynamic models (network of resistances and capacitances analogous to electrical networks) to obtain the heat loss coefficient (HLC) from a dynamic test campaign carried out in an in-use building. It is a well-insulated building located in Gainsborough, U.K. This case study and data were made available to participants in the IEA–EBC Annex 71 project Building Energy Performance Assessment Based on In-Situ Measurements. The analysis reported in this paper is mainly focused on the identification of the main heat transfer contributions and also on the translation of these phenomena to the RC models used to obtain the required HLC. First pre-processing and qualitative analysis were carried out. Afterwards several candidate models were constructed according to different plausible assumptions and approximations. The validity of the results obtained using these models has been evaluated taking into account the agreement among different data series and also the levels of the residuals obtained using the different models. The work concludes obtaining accurate estimates of the HLC from the energy balance including the following relevant contributions: space heating, solar gains, internal gains due to appliances, and internal gains due to metabolic activity. These terms were modelled using the following driving variables: consumption of gas and water, electricity production by the photovoltaic (PV) panels and electricity consumption (modelling internal gains due to appliances and occupancy patterns).

Numerical Simulation of Heat Transfer between Roll and Slab under Dry Secondary Cooling in Ultrathick Slab Continuous Casting

A 3D model of heat transfer between the circular‐distributed water channel roll (CDWCR) and slab during ultrathick slab continuous casting process is established, considering the structure of water channels inside CDWCR and rotating contact between the roll and slab. The results show that the contribution of conduction heat transfer in the heating process of the roll is greater than that of radiation heat transfer, and maximum surface temperature of the roll is 460 K, located at the place where it is right out of contact with the slab, and the change of internal temperature of the roll lags behind the change of surface temperature. The temperature decreases gradually from the outside to the inside in the radial direction. The temperature of the CDWCR inside the circle of cooling water channels is no more than 325 K. The yield strength of CDWCR is analyzed, showing that the strength of the roll meets the requirements of using it in the process. In addition, the orthogonal method is used to analyze the influence factors on CDWCR temperature. The results indicate that the orders of factors affecting more are the contact angle, the slab temperature, the casting speed, and the inlet speed of cooling water.

Thermal Energy Transport across the Interface between Phase Change Material n-Heneicosane in Solid and Liquid Phases and Few-Layer Graphene

Molecular dynamics simulations have been performed to investigate the mechanism of thermal energy transport at the interface between n-heneicosane in solid and liquid phases and few-layer graphene at different temperatures under two heating modes (in the “heat-matrix” mode, heat is flowing from the heated heneicosane molecules to the cooled ones through the graphene layers and in the “heat-graphene” mode, the energy is flowing from the heated graphene to the cooled heneicosane). The effect of orientation of the perfect crystal structure (heneicosane molecules are positioned perpendicular and parallel to the graphene basal plane) on the interfacial thermal conductance has been examined. It is observed that the interfacial thermal conductance is 2 orders of magnitude higher under the heat-matrix mode than under the heat-graphene mode, for liquid or solid heneicosane and monolayer graphene. With an increase in the number of graphene layers, the interfacial thermal conductance under the heat-matrix mode decreases and reaches a plateau when the number of the graphene layer is more than eight. This is caused by the decreasing contribution of direct heat transfer from the matrix to matrix across the graphene layers via nonbonded intermolecular interactions. The interfacial thermal conductance becomes similar for both heating modes, once the number of graphene layers in the system is over 15. The influence of temperature on the interfacial thermal conductance is found to be insignificant in the range (175–250 K; 350–400 K). Both the phase and structure of heneicosane significantly influence the interfacial conductance. Spectral analysis suggests that graphene vibrational modes of all frequencies contribute to the interfacial heat transfer.

Methodological Characteristics of an Experimental Investigation of the Process of Evaporation of Suspended Liquid Droplets

A comparative analysis of the components of a heat flux occurring in evaporation of various liquid droplets suspended on fibers from different materials (polyvinylchloride, asbestos, optical fiber, and horsehair) blown with dry air is presented. The contribution of free convection, radiated emission, and heat transfer through a fiber supporting an evaporating liquid droplet to an integral heat flux on its surface has been determined. The effect of the droplet size, the temperature and velocity of air flow, and the dimensions of suspension and its material on the relationships of the heat flux components on the droplet′s surface has been studied.

Evaluation of bed-to-tube surface heat transfer coefficient for a horizontal tube in bubbling fluidized bed at high temperature

Experiments have been conducted in a bubbling fluidized bed to determine the bed-to-tube surface heat transfer coefficient to a horizontal tube at high bed temperatures (400–950 °C). The aim was to study the influence of the bed temperature, the particle size, the bed material and the superficial gas velocity. A tube (do = 6 mm) through which water is flowing was placed close to the middle of the bed with a fixed bed height of 17 cm. Silica sand, crushed and beneficiated ilmenite and ground steel converter slag were used as bed materials. Air was used as fluidization gas. The mean particle diameter of the used bed materials was in the range 123–327 μm. The estimated heat transfer coefficients were compared with correlations from literature describing the convective bed-to-tube heat transfer coefficient (all of which have been derived at temperatures below 400 °C), with the addition of the radiative heat transfer contribution. The heat transfer coefficient increased with increasing bed temperature, decreasing particle size and increasing superficial gas velocity up to a certain value. These trends were observed for all bed materials although significant differences were still observed in the estimated heat transfer coefficients. Ilmenite was the material for which the highest heat transfer coefficient was estimated. The experiments verified high heat transfer coefficients (768–1858 W/(m2K)) to the outside tube surface operating at a superficial gas velocity of 0.15 m/s, which aligned very well with two of the heat transfer correlations and reasonably well with a third. The remaining three correlations were not as accurate at the examined conditions. The contribution from the convective heat transfer is significantly higher than the estimated radiative heat transfer to the tube.

3D numerical study of natural convection heat transfer from a hollow horizontal cylinder placed on the ground

In the present work, natural convection heat transfer from a thick hollow horizontal cylinder placed on ground is studied numerically. 3D numerical simulations have been performed by varying the Rayleigh number (Ra) from 104 to 108 (preferably within laminar regime). All the simulations have been carried out by varying length to outer diameter ratio (L/D) and thickness ratio (d/D) in the range of 0.2–20 and 0.4–0.9 respectively. Nusselt number, heat transfer rate and contribution to total heat transfer rate from different surfaces have been calculated and their variation with the above parameter is represented graphically. Similarly, thermal plume and velocity vector are shown pictorially at different sections of the cylinder and physical phenomenon of fluid flow and heat transfer could be described from such pictures. Later on effect of above parameters on cooling time is also discussed. It has been observed that with increase in thickness, Nu decreases at low L/D (L/D ≤ 2), whereas at higher L/D a reverse trend is observed. Heat transfer rate from the inner surface increases initially with increase in L/D and after a particular L/D, it attains constant value whereas for outer surface, it increases continuously. It has been also observed that at a particular L/D and d/D, contribution of heat transfer rate from inner surface increases with Ra, whereas reverse trend is observed for outer and side flat surface. A correlation for Nusselt number as a function of Rayleigh number, L/D and d/D has been developed which may be useful for industrial purposes.

Pulsating Heat Pipe Simulations: Impact of PHP Orientation

The pulsating (called also oscillating) heat pipe (PHP) is a simple capillary tube bent in meander and filled with a two-phase fluid. We discuss numerical simulations of the 10-turn copper-water PHP under vertical favorable (bottom-heated), vertical unfavorable (top-heated), and horizontal orientations. Within the present approach, the horizontal orientation is equivalent to the microgravity conditions. The simulations are performed with the in-house CASCO software. The time-averaged spatial distribution of the liquid plugs inside the PHP is influenced by gravity. This affects the overall PHP performance. We show that, independently of the PHP orientation, contribution of the latent heat transfer is large with respect to the sensible heat transfer. We discuss the phenomena occurring inside the PHP during startup and the stable regimes (intermittent and continuous oscillations followed by dryout