Overall Heat Transfer Coefficient Research Articles

New quantitative characterizations of flow and heat transfer in hot dry rock fracture

The overall heat transfer coefficient (OHTC) of rock fractures is a fundamental parameter for characterizing the heat transfer behavior of rock fractures in hot dry rock (HDR) geothermal mining. Although a number of practical formulas for heat transfer coefficients have been developed in the literature, there is still no widely accepted analytical solution. This paper constructs highly accurate analytical solutions for the temperatures of the inner fracture wall and the fluid. Then they are employed to develop new definition-based formulas (formula A and its simplification formula B) of the OHTC, which are well validated by the experimental and numerical simulation results. An empirical correlation formula of heat transfer coefficient is proposed based on the definition-based formulas which can be directly used in the numerical simulations of heat transfer in rock fractures. A site-scale application example of numerical simulation also demonstrates the effectiveness of the empirical correlation formula.

Experimental study on convection heat transfer properties in rough-walled fractures of granite: The effect of fracture roughness

The fracture-dominated convection heat transfer behavior is commonly involved in the development, utilization and storage of thermal energy in fractured rock engineering. An experimental system assembled by a peristaltic pump drive, a liquid preheater and an electric blast drying oven is developed to quantify the effect of fracture roughness on the convection heat transfer characteristics. The overall heat transfer coefficient (OHTC) and the amount of heat transfer quantity from six fracture samples with different inlet temperatures and flow rates are calculated by the data acquisition at five observation points. In general, the average convective heat transfer efficiency between water and rock decreases gradually with time, and then enters a stage of thermal equilibrium while the temperatures at the five observation points become constant. The increasing flow rate can lead to the gradual increase of the OHTC and the slowdown of its growth rate. The OHTC is negatively correlated with the inlet temperature. With the increase of fracture surface roughness, the dominant flow effect is significantly enhanced, which leads to the weakening of heat transfer characteristics and the gradual reduction of OHTC. Finally, the heat transfer quantity decreases with the increase of roughness, and exists an inflection point with the flow rate.

Computational fluid dynamics of flow boiling and conjugate heat transfer characteristics in a mini/micro-channel printed circuit steam generator

Numerical investigation of printed circuit heat exchanger PCHE was carried out for the water-steam system to optimize the design parameters for the possible use of PCHE as a steam generator in integral nuclear reactors- thus targeting efficient renewable energy by enhancing heat transfer. Heat fluxes, overall heat transfer coefficient (OHTC), and volume fraction of steam in a single channel of PCHE are reported for counter-current flow arrangement. The numerical model consisted of 1mm∗1.5mm single hot and cold channel. The heat transfer parameters were calculated using the Volume of Fluid (VOF) model in ANSYS Fluent with boundary conditions: top/bottom adiabatic; left/right periodic; mass flow inlet and pressure outlet. The effect of cold side degree of inlet subcooled (DISC) – varied from 100 to 0 K with a decrement of 20 K- and mass flux (200 to 500 kg/m2.s on OHTC, vapor fraction, pressure drop, and effectiveness was studied. For the fixed degree of subcooled, the OHTC and pressure drop increase as cold side mass flux increases. Similarly, for the fixed mass flux at varying DISC, the OHTC increases and pressure drop decreases for low mass fluxes up to 200 kg/m2.s because most of the channel remains in the nucleate boiling regime. However, with further increase in mass flux opposite trends in OHTC and pressure drop were observed due to single phased regime. The vapor fraction of steam increases with lower DISC at low mass fluxes and decreases with the increase in mass flux. The effectiveness decreases as the cold side inlet temperature approaches saturation temperature at 3 MPa.

Comprehensive numerical investigation of the effect of various baffle design and baffle spacing on a shell and tube heat exchanger

This article explores the significance of Shell and Tube Heat Exchangers (STHX), with a focus on various baffle types such as Segmental, Double segmental, Helical, Disc-and-Doughnut, Clamping anti-vibration, and Flower-B. In this study, the commercial software ANSYS FLUENT is used to simulate the problem. The study evaluates their impact on heat exchanger performance using parameters like overall heat transfer coefficient (OHT), pressure drop (PD), and Performance Evaluation Criteria (PEC). Clamping anti-vibration, Flower-B, Helical, and Double segmental baffles exhibit reductions in shell side PD by 14%-28%. Despite OHT reductions in these designs, Disc-and-Doughnut baffles demonstrate a higher OHT (21%-24%) compared to Segmental baffles. Based on PEC values, Disc-and-Doughnut baffles are identified as the most favorable. Further investigation into different numbers of Disc-and-Doughnut baffles reveals increased OHT (17%-29%), PD (58%-59%), and cold water temperature (0.5%-1%). The STHX with Disc-and-Doughnut 10 baffles displays the highest PEC value, establishing it as the most effective STHX in the simulation.

Study on Flow and Heat Transfer in Single Rock Fractures for Geothermal Heat Extraction

A full understanding of the fluid flow and heat transfer behaviors within a single fracture is important for geothermal heat extraction. In this study, models of single fractures with varying aperture and inner surface roughness (characterized by fractal dimension) are constructed, and a compound fracture aperture (CFA) is proposed to describe the coupled effect of fracture aperture and inner surface roughness. The effect of the fluid flow Reynolds number on heat transfer was investigated as it ranged from 4.84 to 145.63. The results show that the overall heat transfer coefficient (OHTC) in a single fracture significantly increases with the rise in fluid velocity and the compound fracture aperture. Particularly, the OHTC in a single fracture with an inner surface fractal dimension of 2.09 can be up to 1.215 times that of a parallel flat fracture when the flow velocity reaches 0.18 m/s. Moreover, for a fracture with a smaller CFA, enhancing the fracture aperture plays a decisive role in increasing the OHTC. Aperture emerges as a more sensitive optimization parameter for efficient heat extraction compared to the flow velocity. Meanwhile, based on simulation results, a convective heat transfer correlation equation is derived to provide more accurate estimates of the OHTC in rock fractures with different geometries and morphological features.

Optimizing the effect of micro-surface on the thermal hydraulic performance of plate heat exchanger

This research used electrochemical etching as a new method for generating micro surfaces in an effort to improve the thermal–hydraulic performance of a plate heat exchanger (PHE). This technique has significant advantages in terms of producing evenly structured surfaces and enabling exact size control compared to other techniques such as sandblasting and electroplating. Thus, it has great potential to be a pioneer in the industrial implementation of micro surface on heat exchangers. This work is the first to examine the impact of optimum roughness on both the Nusselt number (Nu) and the Performance of Evaluation Criterion (PEC) of PHE using a multi-objective genetic algorithm. In order to do this, several microstructure surfaces were generated on the PHEs, with average roughness values ranging from 0.65 to 1.51 μm. The impact of these microstructure surfaces on heat transfer and pressure drop was experimentally investigated through heat transfer experiments conducted at Reynolds numbers (Re) ranging from 3,000 to 12,500. The findings reveal that the introduction of microstructure surfaces leads to a noteworthy enhancement of 10 % to 18 % in the overall heat transfer coefficient (OHTC) across all experimental scenarios. Conversely, the friction factor experienced an increase of 28 % to 53 % compared to a smooth surface. PEC values ranged from 1.05 to 1.21, showing a superior heat transfer performance over a marginal pressure drop increase. Furthermore, the microstructure surfaces demonstrated significantly improved values of 0.23 to 0.32 for the Number of Transfer Units (NTU) and 0.18 to 0.26 for effectiveness, in comparison to the smooth surface. This corresponds to an enhancement of 9.7 % to 18.5 % and 7.2 % to 16.4 %, respectively. In addition, a multi-objective genetic algorithm determined that an optimal roughness value of Ra = 1.1 μm achieves a balance between maximizing Nu and PEC.

Forecast Study on the Overall Coefficient for Passage of Single-Phase and Passage of Single-Phase in an Annular to Plain Tubes Heat Stream

Abstract To prevent buoyancy effects from increasing the overall heat transfer coefficient due to low heat flux, smooth vertical circular tubes require forced convection conjectures. Previous research has focused on mixed convection in turbulent regions and has limited studies on forced convective heat transfer. This study aims to investigate the predictable overall coefficients of heat transfer performance under specific conditions of forced convection, considering both vertical up flow and down flow through the smooth circular test section. The recommended Reynolds numbers for water are (7201.6–14577.4) and (3186.7–6396.75) for single-phase and single-phase annular passages, respectively, with a heat application range of (50–500 W). The overall coefficient of heat transfer is (333.91 - 432.95 W/m2.K) and (169.76 - 251.20 W/m2.K) for single-phase and single-phase annular passages, respectively. The flow was turbulent for all heat fluxes.

Experimental investigation of heat transfer characteristics for a shell and tube heat exchanger

Abstract In the present work, numerical investigations are conducted with 22 % cut segmental baffle heat exchanger (SB), 20°, 30°, and 40° helical baffles shell and tube heat exchangers (STHX) to estimate the overall heat transfer coefficient (OHTC), pressure drop (PD) and friction factor. Among the studied heat exchangers (HE), 40° helical baffles STHX provided the highest OHTC with minimum pressure drop. Hence, further investigations are conducted experimentally with 40° helical baffles STHX. OHTC increased by 2.65 % for 20° helical baffles, 5.37 % for 30° helical baffles, 9.78 % for 40° helical baffles when compared with 22 % cut segmental baffle heat exchanger. The deviation between experimental and numerical OHTC is 2.64 % 40° helical baffles.

Experimental investigation of an automobile radiator using carbon based hybrid nano coolant

Comprehensive experimentation on the heat transfer performance of a vehicle radiator with a novel hybrid nanofluid (HyNf) comprising Graphene nanoplatelets (GnP) and Multi-walled carbon nanotubes (MWCNT) nanoparticles in 1:1 ratio, dispersed in radiator coolant oil (base fluid), has been performed. Non-covalent functionalization method was implemented to prepare steady nanofluids for volume concentrations of 0.05%, 0.1%, 0.15%, 0.2%, 0.3%, 0.4%, and 0.5%. Tests were conducted for different nanofluid flow rates from 20 g/s to 100 g/s in 5 steps, three different nanofluid entry temperatures of 30 °C, 50 °C and 70 °C and three different air velocities viz. 1, 3 and 5 m/s. The enhancement in thermal conductivity (TC) was observed as 59% for 0.5 vol % compared to base fluid, which leads to an augmentation in the convective heat transfer coefficient (CHTC) of nanofluid. The maximum improvement in overall heat transfer coefficient (OHTC) was found as ∼ 124% at 0.5% volume concentration, 5 m/s air velocity, compared to the base fluid. A rise in the pressure drop is found as 23.72% compared to base fluid. Due to these enhanced heat transfer behaviours of the synthesized GnP-MWCNT/coolant oil HyNf, it can be an alternate to the existing coolant oil.

Hollow fiber polytetrafluoroethylene membrane heat exchanger with anti-corrosion properties

Thin hollow polytetrafluoroethylene (PTFE) fibers (thickness: 200 μm; outer diameter: 1.3 mm; inner diameter: 0.9 mm) were produced by a specially designed die via stretching and sintering, and assembled into a shell-and-tube membrane heat exchanger. The effects of the flow velocities on the heat transfer performance and thermal resistance were studied in a water-water system. The maximum overall heat transfer coefficient (OHTC) was up to 391 W/(m2‧K) (flow velocities: 0.24 m/s for hot water; 0.11 m/s for cold water). The OHTC became larger (increased from 229 to 391 W/(m2‧K)) at a higher cold water flow velocity (increased from 0.02 to 0.11 m/s) on the shell side when maintaining the hot water flows on the tube side. The OHTC increased from 119 to 391 W/(m2‧K) first and then decreased to 298 W/(m2‧K) with the increase of the hot water flow velocities (from 0.08 to 0.40 m/s) on the tube side, while maintaining the cold water flow velocity on the shell side. The thermal resistance results showed that the increased flow velocity (0.11 m/s) on the shell side significantly reduced the thermal resistance to 2.56 × 10−3 K m2/W on the shell side. Furthermore, the flow turbulence on the tube side is favorable for the heat transfer, and the optimum flow velocity on the tube side was 0.24 m/s. The hollow PTFE fibers were extremely stable and durable in corrosive solutions, with less than 1% changes in bursting strength, tensile strength at break, and elongation at break. This work opens new opportunities for the fabrication and application of hollow fiber PTFE membrane heat exchangers to reuse waste heat in high temperature corrosive environments.

The effect of minichannels on the overall heat transfer coefficient and pressure drop of a shell and tube heat exchanger: Experimental performance comparison

Channels with small hydraulic diameters improve the overall heat transfer coefficient (OHTC) and compactness of shell and tube heat exchangers (STHEs). Intensified compactness decreases both the weight and volume of heat exchangers and allows us to use less working fluid. This study experimentally investigated the effects of minichannels on the OHTC and pressure drop performance of a STHE for five different thermal and hydrodynamic operating conditions. Present study also compared the experimental OHTC and pressure drop results of the mini-channel shell and tube heat exchanger (MC-STHE) with those of previously reported experimental results of conventional STHEs under different conditions. The MC-STHE had an OHTC of 1.1–6.6 times as high as the conventional heat exchangers. Besides, the minichannels increased the OHTC of the STHE to ∼6700 W/m2K and yielded a higher pressure drop than the macro tube heat exchangers at Re ≅ 14,000 (∀˙ = 600 L/h).

Study on performance of working model of heat exchangers

The most often utilized heat exchangers are tubular heat exchangers. Kern Method is one of the ways for thermally designing a shell and tube heat exchanger. A small scale 1, 2-shell and tube heat exchanger is fabricated after designing, in this paper. The experimental value of the overall coefficient of heat transfer is validated with the experimental value computed using the Kern Method, as well as to values found in the literature. The effects of tube outer diameter and tube thickness on the various parameters of a 1–2 tubular heat exchanger were investigated. The fluctuation of coefficients of heat transfer on the tube and shell sides, Reynolds numbers, drop in pressure, and overall heat transfer heat transfer coefficients are found.

Numerical study on novel airfoil corrugated plate heat exchanger: A comparison with commercial type and geometrical parameter analysis

• Symmetric airfoil is used for PHE design for the first time. • CFD simulation is validated with a previous experimental. • New type of PHE has less pressure drop than commercial type while maintaining the same amount of heat transfer. • Effects of geometrical parameters of plate heat exchanger on performance was numerically studied. • Airfoil PHE is a potential alternative to commercial type. Utilizing an effective corrugation shape in plate heat exchanger (PHE) results in a substantial increase in thermal performance and a more compact design for these devices. This article presents, for the first time in the literature, an innovative PHE design that employs symmetric airfoil profile corrugation (airfoil corrugated PHE). As a consequence of the airfoil corrugation, unique flow channel is formed inside the PHE, which results in lower pressure drop while maintaining the equivalent heat transfer rate as the commonly used commercial sinusoidal PHE. In order to get a comprehensive understanding of the performance of airfoil corrugated PHE, essential parameters such as corrugated angle, corrugated pitch, mirror arrangement, and airfoil profile were examined over Reynolds number ranges of 4,600–8,400. Compared to sinusoidal corrugation, the innovative PHE has roughly 17% reduced pressure drop while keeping the same amount of heat transfer, as determined by a computational analysis of thermal and hydraulic parameters. 60° was the optimal corrugated angle, demonstrating a 10% increase in overall heat transfer coefficient (OHTC) compared to both 30° and 90°. In addition, the NACA 0030 airfoil profile obtained the highest thermal performance, with an OHTC that was roughly 12% greater than that of the NACA 0020 and NACA 0025 profiles. Nonetheless, as one of the geometrical parameters changes, the pressure drop is proportionate to the increase in heat transfer. NACA 0030 saw a pressure drop increase of around 33.7%, the greatest of any case. The geometrical parametric study indicates that the ideal configuration of the innovative PHE consists of NACA 0030 airfoil profile, chevron angle of 60° degrees, single mirror arrangement, and corrugated pitch of 3.14 mm, as shown by the maximum performance of evaluation criterion (PEC) value of 1.08.

Potential evaluation of water-based ferric oxide (Fe2O3-water) nanocoolant: An experimental study

Fuel combustion generates a mammoth extent of heat in the engine's combustion chamber. The high temperature of combustion gases makes the engine prone to seizure. To alleviate the severity of overheating, efficient heat removal is needed. In the current research, the potential evaluation of low concentration Fe2O3/water nanocoolant for automotive cooling has been performed experimentally. Three concentrations (0.003, 0.005, and 0.007 vol%) of Fe2O3/water nanocoolant with three different inlet temperatures (45, 50, and 55 °C) and five flowrates (10–14 LPM) were opted to appraise the performance of nanocoolant based automotive cooling system. As per the experimental results, the topmost enhancement in heat transfer rate was observed to be 21.89% for 0.007 vol% Fe2O3/water nanocoolant as compared to water. Furthermore, a 20.65% hike in convective heat transfer coefficient (CHTC), 18.24% escalation in Nusselt number, and 19.51% augmentation in overall heat transfer coefficient (OHTC) were obtained for the nanocoolant of the aforementioned concentration. All the parameters were observed to mount with mounting concentration and flowrate of the nanocoolant, however, the inlet temperature of the nanofluid did not produce significant effects. Results of the current research are exceedingly encouraging since an inordinate enhancement in performance parameters has been observed even at a low concentration of nanocoolant.

Temperature effect on thermal-hydraulic performance of one-pass counter-current flow shell-and-tube heat exchanger and upon its design

A methodology of design and analysis of thermal-hydraulic performance for a single pass 1-1 counter-current flow shell and tube heat exchanger (CCFSTHE) TEMA E-type has been established. The temperature effect on the thermo-physical properties of flowing fluids and on the overall coefficient of heat transfer along the heat exchanger is incorporated in our approach, as well as the coupling between different thermal and hydraulic parameters. It has been noted that the correction factor (F) in the HAUSBRAND formula is not included. Our method brings us to a new dimensionless quantity (MKA) which links the calculation parameters of the heat exchanger to the thermo-physical properties. This dimensionless quantity relates the number of transfer units to the heat flow ratio, R. The results based on our models show a pronounced deviation compared to the model reported in the literature (number of transfer units method). This deviation may be related to a temperature effect not included in the literature model. It has been shown that the results derived from our models are in a good agreement with experimental data. Our new method, named MKA - method, could be a useful tool for theoretical and experimental studies of the design and analysis of the single pass 1-1 CCFSTHE thermal and hydraulic performance for 0 ? R ?1.

Experimental analysis on heat exchanger using Al2O3-CuO hybrid nano coolant

This research work gives the insight on heat transfer performance of nano coolants using hybrid nanoparticles Al2O3-CuO in mixture ratio 20:80, 50:50, 80:20 respectively, at 0.1 vol% of 5 liter base fluid (mono ethylene glycol (MEG)+ deionized water (DW)) on a double pipe shell and tube heat exchanger. Nanoparticles were prepared using ball milling, magnetic stirring and ultra-sonication. The average diameter of crystallites of hybridized nanoparticles was in the range of 25 nm to 35 nm, determined using an X-ray diffractometer. Specific heat capacity, density and kinematic viscosity were determined using differential scanning calorimetry, Redwood viscometer and precision balance, respectively. Modifications were done on the cold side of the heat exchanger by installing a tank at the top for the nano coolant. The flow rates on cold fluid side was fixed to 1.2 L/min and hot fluid side varied from 0.6 L/min to 1.2 L/min operated on both, parallel flow and counter flow arrangement. Coolant sample 80:20 (Al2O3:CuO) amongst all the samples possessed highest rate of effectiveness with an increment of 35% when compared to traditional sample DW + MEG and also with water. Highest logarithmic mean temperature difference (LMTD) and overall heat transfer coefficient (OHTC) were observed for the above sample. Smidgen amount of agglomeration had been observed while conducting the experiment.

Thermodynamic and hydraulic design characteristics of the fin tube heat exchanger

Heat exchangers are crucial devices that are used to exchange heat between two different fluids: one that is hot and one that is cold. Amongst them, finned tube heat exchangers play a vital role in today’s thermal industry. Due to the increased surface area of the tubes' outer surface (or fins) when utilizing finned tube heat exchangers, the heat transfer rate may be improved as a consequence of the increased surface area of the fins. It is often referred to as a compact heat exchanger in order to highlight the comparatively large heat transfer surface area to volume ratio of the exchanger. They have many vital applications in the field of refrigeration, air conditioning, IC engines, and power plants. The fin and tube heat exchanger has been analyzed for three different cases in the present study: preheater, condenser, and evaporator. The Engineering Equation Solver (EES) software has been used to analyze the heat exchanger for the various thermal, hydrodynamic, and design parameters. Results have been plotted by comparing the various thermal parameters such as overall and inside heat transfer coefficient, Nusselt number, and rate of heat transfer with respect to Reynolds number. Similarly, the hydrodynamic parameters such as Colburn factor, friction factor, and the design parameters such as the number of fins and fin spacing.

Experimental study of convective heat transfer characteristics of fractures with different morphologies based on fractal theory

In the Enhanced Geothermal System (EGS), the flow state of the fracture of the thermal reservoir (granite) is important for heat transfer. In this paper, the effects of reservoir temperature, flow rate and fracture shape on the thermal convection that occurs in the fracture of rough granite are studied based on the fractal dimension theory. The rough fracture is manufactured regularly by carving technology. The shear flow experiments were carried out at confining pressures of 10, 15 and 20 MPa to analyze the flow state. The convective heat transfer experiments were conducted at 60 and 70 °C to simulate different heat reservoir conditions. The results indicated that the outlet temperature and overall heat transfer coefficient (OHTC) increased with the increase in the rock temperature. A higher flow rate can enhance the thermal performance. The complexity of the fracture surface is also an important factor that affects the convective heat transfer process. The larger fractal dimension of the fracture surface can more easily excite turbulence, affecting the thickness of the boundary layer. The OHTC will increase as the fractal dimension increases.

A detailed hydrothermal investigation of a helical micro double-tube heat exchanger for a wide range of helix pitch length

The present study was numerically inquired the heat transfer performance and fluid flow characteristic of a helical micro double-tube heat exchanger (HMDTHX) using the finite volume method. The tube length was considered to be constantly equal to 30 mm, and 12 different configurations were modeled by changing in turn number and pitch length (P) for Reynolds numbers of 50, 100, 150, and 200. The findings indicated that the heat transfer would enhance by applying any helix angle in the straight tube. However, it had an optimum point which varied by Reynolds number (Re). Rising Re caused overall heat transfer coefficient (OHTC), pressure drop, and pumping power augment for all cases. Increasing P in overall reduced OHTC, pressure drop, and pumping power which had different maximum points between P = 0.5 to 3. Maximum overall heat transfer coefficient (OHTC) enhancement was equal to 45% for Re = 200 and P = 2. Also, maximum effectiveness was 11.5% for P = 2 and Re = 200. Moreover, a 42% maximum increment was achieved for pressure drop, pumping power, and friction factor at Re = 200 and P = 2. Shear stress for Re = 100 to 200 showed that the values are almost the same for P = 0.5 and 1. Then by increasing P, the shear stress decreases. While, for Re = 50, a maximum is seen at P = 2. The temperature distribution was indicated that the maximum temperature of the straight tube and helical tube are the same, but the difference is in the average temperature, which was 3.2 K between straight and helical tubes. Finally, by investigating the velocity contour, it was determined that a secondary flow through the HMDTHX, affected by centrifugal force, was existed, enhancing the fluid flow turbulency and heat transfer rate.

Analytical and numerical study on heat transfer coefficient between rock fracture walls and flowing fluid

The heat transfer coefficient (HTC) is an important parameter describing the heat transfer characteristics between rock fracture walls and flowing fluid. This study summarizes and evaluates the existing typical overall heat transfer coefficient (OHTC) formulas (A - D) and proposes four new OHTC formulas (E - H). As a novelty different from the existing analytical methods, the new method solves the temperature distribution of a cylindrical rock sample in the form of integral, and thus obtains the expression of fracture surface temperature. Various experimental data were employed to evaluate each formula. The adaptability and stability of OHTC formulas are closely related to the flow rate. Based on the commonness of each formula, the key parameter affecting OHTC value is presented. Fluid flow and heat transfer in a single fracture is numerically modeled. Comparison of outlet water temperature obtained by numerical method and experimental measurement shows that the accuracy of formulas D, G and H is higher. For comparison with the parallel-plate model, the fractures with heterogeneous aperture are constructed, and heterogeneous HTC is adopted. The results show that the water temperature contours in fractures modeled with homogeneous HTC are more tortuous, and the temperature contours modeled with heterogeneous HTC are smoother. As the flow rate increases, the difference of outlet temperature between fractures with homogeneous and heterogeneous apertures increases.