Side Heat Transfer Research Articles

Experimental study and comparative performance analysis on thermal-hydraulic characteristic of a novel longitudinal flow oil cooler

The longitudinal flow heat exchanger possesses lots of merits but it exhibits poor heat transfer coefficient at low Re condition, which makes it difficult to be applied in the high viscosity fluid area. In this paper, a heat transfer augment solution for lubricating oil flowing on shell side of a novel longitudinal flow twisted trifoliate tube oil cooler was proposed. Experiments were conducted to comparatively investigate the thermal–hydraulic performance of a twisted trifoliate tube oil cooler and a conventional segmental baffle oil cooler. The tube side study demonstrates that the Nu and f of the twisted trifoliate tube are 17.5–75.8% and 16.9–62.9% larger than that of the smooth circular tube. The shell side study shows that at the same oil mass flow rate, the shell side heat transfer coefficient of the twisted trifoliate tube oil cooler is 66.3–148.9% higher than that of the conventional segmental baffle oil cooler with pressure drop increased by 31.6–65.2%. Furthermore, the shell side performance evaluation criterion h/ΔP of the twisted trifoliate tube oil cooler is 1.46 times larger than that of the conventional segmental baffle oil cooler on average. The possible mechanisms responsible for the heat transfer enhancement are analyzed. In addition, the predictive correlations of tube side and shell side of the twisted trifoliate tube bundles for Nu and f are deduced based on the experimental data. Results from current research could provide useful correlations and beneficial guidance for design and application of the longitudinal flow heat exchanger in low Re ranging of 40–250.

Experimental and Mathematical Research of Convection Heat Transfer for a High Integral Finned Tube Heat Exchanger

This research exhibits an experimental and numerical investigation for the characteristics of heat transfer by utilizing an integral and smooth high finned tube and shows how the integral high fins affect in improving the transfer of heat. In the experimental work, the experimental rig consists of a cold water loop, a hot air loop and the trial part which is a concentric parallel flow double pipe heat exchanger. The first test smooth tube made of brass has external and internal diameter (32 mm) and (22 mm), respectively, the second test section made of the same material has external fins, the third test section possesses internal fins, and the fourth test section has internal and external fins. The dimensions of the fin are (2 mm) in thickness, (2 mm) in height and pitch (2.5 mm) center to center. And, the water flow rates are (6, 8, 10, 12 and 14 L/min). The inlet water to the trial tube was at temperatures (15, 25, 35, and 45oC). The investigational results manifested that the air side heat transfer coefficient of the smooth tube was lesser than the integral high finned tube. The ratio of improvement when using the integral high finned tube was (47.5%, 60.5% and 67.5 %). Numerical simulation was applied on the current heat exchanger to study both the transfer of heat and the field of flow by using ANSYS, FLUENT15 package. Steady state, Newtonian flow, incompressible and three dimensional analyses were assumed. The comparison between experimental work and numerical results elucidated a good agreement.

Numerical Simulation of Heat Transfer Tube Characteristics of Refrigeration and Air Conditioning Based on Mixed Fractional Model

The main parameters affecting the heat transfer performance of heat transfer tube heat exchanges include fin shape, fin spacing, fin thickness, tube row arrangement, tube diameter, dry and wet bulb temperature and flow rate. The air side heat transfer performance of heat transfer tube heat exchange and the influence of velocity field and temperature field distribution on heat transfer effect have been the focus of domestic and foreign scholars. In this paper, based on the mixed fraction model, CFD software is used to simulate the absorption process of gravity falling film outside the heat transfer tubes of refrigeration and air conditioning, and to study the flow and heat transfer characteristics of the process. The results show that, for the heat transfer tubes with the selected structure, the heat transfer capacity increases with the increase of water flow velocity, and the heat transfer enhancement effect of turbulence is enhanced. The heat transfer tubes have better comprehensive heat transfer performance than smooth tubes with the same diameter.

Thermal and flow characteristics of a channel formed by aligned round tube bank fins stamped with curve delta-winglet vortex generators

The primary purpose of present study is to reveal the roles played by CDVGs in vectoring the airflow and breeding longitudinal vortices for enhancing heat transfer. The thermal-hydraulic characteristics of a finned aligned round tube heat exchanger with stamped CDVGs were numerically studied by using the custom code with SIMPLE algorithm in a nonorthogonal curvilinear coordinate system. Four geometric parameters including vortex generator height, radial position, peripheral position and fin pitch, were programmed to ascertain parametric effects on fin side thermal performance, and the regression formulas of Nusselt number and resistance coefficient in the fin side passage with Reynolds numbers and four geometric parameters were secured. The findings suggest that the mean Nusselt number, resistance coefficient and secondary flow strength for the novel fin with CDVGs are all prevail over that in the reference plain fin under a certain Reynolds number. The body fitted CDVGs are ideally situated for finned round tube exchanger in aligned arrangement. The action of CDVGs can not only efficiently lessen the size of recirculation area behind tube but also produce strong longitudinal vortices, accordingly heighten evidently the fin side heat transfer ability in the recirculation zone. Using the studied optimum geometric parameters can make the thermal property factor up to 1.35 under the same pumping power constraint in comparison with the reference plain fin geometry when Reynolds number equals to 3000. This study can be a real application in compact heat exchanger with finned round tubes when heat transfer augmentation is under consideration.

A comparison study on thermal performance enhancement of corrugated oil cooler with internal turbulators – Numerical and experimental approach

Oil coolers play an important role in dissipating excess heat from engine oil to surroundings. In this work, performance study of an oil cooler with internal turbulators of dimple and offset type is performed using numerical approach and compared with experimental results for validation. In numerical approach, oil cooler with turbulators are numerically modeled using Conjugate Heat Transfer (CHT) approach to solve for heat transfer characteristics. Turbulence physics modeling is done using Reynolds Averaged Navier Stokes equations (RANS) through realizable k-epsilon model with standard wall function. The air side Heat Transfer Coefficient (HTC) estimation is done with CHT model using one corrugation, as the air flow in fins is periodic. In experimental study, Required Inlet Temperature Difference (ITD), Oil Flow Rate, Air Flow Rate is maintained, measurements are recorded to create performance data matrix with combinations of various operating data points for air and oil sides respectively. Numerical results are compared with experimental results obtained; Deviation for heat rejection of oil-cooler with dimple turbulator fall in range of ±7%, offset turbulator fall in the range of −2.0 to 9.4%. Numerical data shows that the thermal performance of offset type is better than dimple type by 10–13.5%, while the test data depicts that performance of oil-cooler with offset turbulator is high by 12–19% than dimple type and thereby proving its performance superiority. In offset turbulator additional turbulent scales of the offset length scale range are generated, these additional scales are evident by increasing the Turbulent Kinetic Energy (TKE) of 46–51% than dimple turbulator from numerical study. Thus, for same plate and fin, oil-cooler with offset turbulator has better thermal performance than oil cooler with dimple turbulator.

Carbonaceous Deposits in a Fuel-Film Cooled Rocket Combustor: Electron Microscopy

The high gas–side heat transfer coefficients expected in high-pressure oxygen-rich staged combustion kerosene–oxygen engines require fuel–film cooling in parallel with regenerative cooling to keep metal wall temperatures at acceptable levels. In such engines, the fuel–film transitions from a liquid to supercritical state, leaving behind dense and soot layers of carbonaceous deposits. We used a 4.8 MPa kerosene– fuel–film cooled combustor with removable chamber liner samples for posttest analysis of carbonaceous deposits. Dense layer deposit depths were measured in cross-sectioned samples with a dual-beam scanning electron microscope. The varying fuel–film flow rate did not affect deposit depths in the near field. However, higher film flow rates resulted in longer regions of carbonaceous deposits in the far field. The dense and soot layers were hypothesized to be formed by heterogeneous condensation of polycyclic aromatic hydrocarbons and thermophoretic diffusion of soot, respectively.

Convection enhancement using composite vortex generator

This paper presents an original concept of using composite flexible flapping vortex generator mounted on a heat sink fin for air side heat transfer augmentation. The main aim is to combine the advantages of hard and soft winglets in a composite one for having the highest possible enhancement. The proposed composite vortex generator, which is made with a thin elastic sheet is responsible for enhancing heat transfer and mixing quality performances in laminar convection air flow in a heat sink. The merged vortical structures due to oscillation by winglet swept out the thermal boundary layer and enhance thermal mixing between the fluid near the heated fin and the channel core flow. This novel concept is demonstrated using numerical simulation of the flow field with considering a two-way strongly coupled fluid-solid interaction approach in transient condition. The set of governing equations including, the continuity, momentum, and energy for a 2-D forced convection air flow are solved by the finite element method using the COMSOL Multi-Physics. The present findings show 148%, 116% and 121% increases in the cooling rate by the composite and the two hard and soft homogeneous winglets, respectively. Numerical results are validated against the numerical data reported in the literature.

Design and testing of energy-efficient heat exchangers for Newtonian and non-Newtonian fluids – A review

Heat transfer enhancement, pumping power and weight minimization in enhanced heat exchangers has long been achieved by deploying tubes with internal surface modifications like microgrooves, ribs, fins, knurls, and dimples with and without tube inserts. This article presents a very extensive review of experimental and computational studies on heat transfer enhancement, which covers convectional and unconventional working fluids under different fluid flow conditions. Compound augmentation with tube surface modifications and inserts has yielded enhancements in the overall heat transfer coefficient of over 116% in the fully developed turbulent flow regime. Exotic fluids like nano-coolants deployed in spiral grooved mircofin tubes yielded 196% enhancement in tube side heat transfer rate for concentrations as low as 0.5% by volume, while the thermal efficiency index measuring the overall enhancement in relation to the pumping power was 75%. However, reviews that address the combined effect of unconventional fluids, surface modifications and tube inserts on the overall thermo-hydraulic performance of annular heat exchangers seem to be limited. Further, nano-coolants aren’t frequently used in the process industry. The goal of this study is to document and evaluate the impact of cost-effective and energy-saving passive enhancement techniques such as tube surface modifications, tube inserts, and annular enhancement techniques on annular heat exchangers used in the process industries with Newtonian and non-Newtonian fluids. This review should be useful to engineers, academics and medical professionals working with non-Newtonian fluids and enhanced heat exchangers.

Enhanced thermal performance of finned latent heat thermal energy storage system: fin parameters optimization

Integration of thermal energy storage (TES) systems in concentrated solar power (CSP) plants plays an important role, followingly, the mismatch between energy production and demand can be adjusted. Generally, latent heat thermal energy storage (LHTES) can ensure important amounts of energy compared to sensible heat thermal energy storage systems (SHTES), which has oriented researchers, engineers, and decision makers toward using this technology because of its high energy density. Otherwise, using phase change materials (PCMs) in LHTES systems can considerably reduce their performance as PCMs have relatively low thermal conductivity values, which makes it very necessary to take measures for enhancing the thermal conductivity or heat transfer inside of these LHTES units. This study simulated, modeled and optimized a proposed system for heat transfer enhancement, which is a shell-and-tube storage units with annular fins, by using the finite element method during the charging and discharging processes. In addition, NaNO3–NaNO2 was used as PCM while the Delco Term Solar E 15 was used as heat transfer fluid (HTF). The main objective of this work is to provide the optimal geometry parameters of the studied system in terms of length, number, step, and fin thickness by comparing many fin geometries. According to the results, an increase around 65.04% in the charging (melting) time and 58.36% in the discharging (solidification) period was achieved by the integration of optimal fins. The recommended fin parameters corresponding to the highest thermal performance were; length = 14 mm, thickness = 1 mm, step = 3.45 mm, and number = 66.

Multi-objective optimization of the generator air cooler based on genetic algorithm

Generator unit No.1 in an electric power plant cannot run at full load owing to the high temperature of generator cooling air. Analysis has proved that the problem lay in the large cooling load and low air side heat transfer coefficient, and thus, the cooling requirement cannot be meet in summer. Therefore, a generator air cooler with circular fin structure is optimized under summer operating conditions. The heat transfer coefficient, air side pressure drop, water side pressure drop resistance and overall weight are used as the optimization objectives using genetic algorithm, to optimize structure parameters of the generator air cooler. Through analysis, the best structure combination can be selected to manufacture a product. The optimized generator air cooler could meet the generator cooling requirements, operate at full load, and also reduce the investment and operation costs.

Colburn vs Bell Delaware approach for evaluation of thermohydraulic performance of a shell and tube heat exchanger: A theoretical study

STHX have assumed importance over the years due to its great versatility. Thermal and hydraulic characteristics dictate the performance of the heat exchanger. These factors are dependent on the inlet conditions, flow conditions, tube and shell geometry. Present paper presents a theoretical approach involved in calculating the thermohydraulic performance of a STHX using two methods- Colburn and Bell Delaware. Temperature of water on the hot side was varied- 50 °C, 70 °C and 90 °C and the cold fluid has been set to 30 °C. Plots of performance parameters such as heat load, shell side heat transfer coefficient, and effectiveness by both approaches have been discussed. From the results it can be concluded that the Bell Delaware is more reliable for calculation in practical scenarios as the losses that may occur are taken into consideration.

A novel spiral channel with the growing waviness on the sidewalls for compact high-efficiency heat exchanger

Spiral channels are widely used as the core heat exchangers in latent thermal storage units. This study aims to develop a novel spiral channel characterized by wavy sidewalls for high-efficiency convective heat transfer in tube side. The amplitude of the wavy sidewalls is designed to grow from the inlet to the outlet along flow direction. The thermal characteristics in the spiral-wavy channel were numerically investigated under the boundary condition of constant wall temperature, since latent heat storage units mainly work under isothermal condition. The comparison was carried out between a smooth-spiral channel and a spiral-wavy channel in terms of the heat transfer capacity, friction factor, dimensionless temperature distributions and development of boundary layers. At a given heat transfer rate, the required length of the spiral-wavy channel is only 63.5% of that of the smooth-spiral channel, which implies that the energy density and efficiency of a heat storage unit can be improved remarkably. The increase in pressure drop brought by the spiral-wave channel is less important compared with the improvement of thermal performance, suggesting its potential in heat transfer enhancement and energy saving. The present work is expected to motivate the design of compact and high-efficiency heat exchangers for thermal energy storage and extraction.

Bridging the Gaps: from Particle-resolved to Multi-tubular Reactor Simulation

Packed bed reactors have been widely used in the chemical and process industry for several decades. They can be difficult to design and operate due to their size and complexity. Therefore, there is still room for improvement of the performance of these reactors and rigorous simulations can help here to achieve desired goals with lower upfront investment. Recent advances in modeling particle-resolved packed beds allows a detailed inside in the flow, species and temperature distribution in the beds and therefore also into the conversion of the surface reactions. Based on these simulations, model parameters for 1D-models can be estimated. These 1-D models can then be used to calculate performance of multi-tubular reactors, either by running a computationally expensive simulation with resolved tubes or by coupling the CFD simulation to an advanced process modeling tool like gPROMS. In the latter case, the flow non-uniformity as well as certain flow properties like coolant velocity and temperature is taken from the CFD simulations, while on the process modeling side heat transfer and reactions in the packed bed are calculated based on the simplified 1D models. This modeling approach is fully 2-way coupled and highly efficient in terms of accuracy and especially runtime and it can be embedded into a flow sheet simulation.In this contribution, we will present the whole simulation process and how the different steps intertwine with each other starting from the detailed particle resolved simulation all the way down to the flow sheet simulation. The benefit of this approach will be demonstrated based on several examples.

Supercooling in a new two-stage thermoelectric cooler design with phase change material and Thomson effect

A novel physical model of a two-stage thermoelectric cooler (TEC) is developed, based on geometry factor (γ) analysis between stages under pulsed current conditions and with phase change material (PCM), to improve performance. A detailed investigation of a two-stage Peltier cooler based on a general thermodynamic formulation considering the Thomson effect is presented. The minimum cold side temperature, coefficient of performance (COP), and further the thermoelement’s characteristic temperature profiles are discussed. The numerical analysis was carried out considering square pulse current, hot side heat transfer, metal strips, ceramic plates, and a PCM material volume in the heat sink. The investigation results proved that the geometry relation between both stages and the Thomson effect directly impacts supercooling, and a higher reduction in the cold side temperature can be achieved compared with conventional thermoelectric coolers. A reduction of 25.2 K in a two-stage over a single-stage TEC with PCM is achieved considering a large cross-sectional area in the first stage. It has been found that the holding time and the characteristic temperature profiles during and after pulse operation depends strongly on Thomson heat and geometry. A shorter holding time and maximum temperature drop are found for γ>1 values due to the PCM’s melting temperature and the higher pulse current. The characteristic cooling length reveals the part of the semiconductor element that is cooled during pulse operation. Results will guide the design or selection of a heat sink crucial to a solid-state cooling device’s overall performance.

Heat transfer modelling of semi-suspension biomass fired industrial watertube boiler at full- and part-load using CFD

In the current work, a computational fluid dynamics model is presented of a semi-suspension fired bagasse boiler which pays special attention to resolving the heat transfer to the furnace walls, superheater tubes and evaporator tube bank. The boiler CFD model includes custom-developed sub-models programmed in the C language which captures the fuel particle grate interaction, cylindrical fuel particle drag effects, superheater steam side heat transfer and evaporator heat exchange using a porous media approach. The model is applied to simulate the case study boiler for 100% and 65% load cases. The CFD model results are verified using a steady-state process model which utilizes well-known empirical heat transfer correlations. The CFD model results are also validated using plant measurements. The results of the CFD model show that it can accurately predict the outlet steam temperatures of the superheaters with relative errors of between 0.89 and 1.6% compared to the site data. The model is also capable of accurately predicting the evaporator outlet gas temperatures with a relative error of approximately 1.4%. Once the model is verified and validated it is used to investigate heat flux and metal temperature distributions in the various heat exchangers. The results show that at part load conditions the combustion occurs much closer to the furnace rear wall due to deeper spreading of the fuel particles, which results in the front wall having a relatively low heat absorption rate compared to the side and rear walls.

Pulsed Impingement Turbine Cooling and Its Effect on the Efficiency of Gas Turbines With Pressure Gain Combustion

Abstract Performance improvements of conventional gas turbines are becoming increasingly difficult and costly to achieve. Pressure gain combustion (PGC) has emerged as a promising technology in this respect, due to the higher thermal efficiency of the respective ideal gas turbine cycle. Previous cycle analyses considering turbine cooling methods have shown that the application of pressure gain combustion may require more turbine cooling air. This has a direct impact on the cycle efficiency and reduces the possible efficiency gain that can potentially be harvested from the new combustion technology. Novel cooling techniques could unlock an existing potential for a further increase in efficiency. Such a novel turbine cooling approach is the application of pulsed impingement jets inside the turbine blades. In the first part of this paper, results of pulsed impingement cooling experiments on a curved plate are presented. The potential of this novel cooling approach to increase the convective heat transfer in the inner side of turbine blades is quantified. The second part of this paper presents a gas turbine cycle analysis where the improved cooling approach is incorporated in the cooling air calculation. The effect of pulsed impingement cooling on the overall cycle efficiency is shown for both Joule and PGC cycles. In contrast to the authors’ anticipation, the results suggest that for relevant thermodynamic cycles pulsed impingement cooling increases the thermal efficiency of Joule cycles more significantly than it does in the case of PGC cycles. Thermal efficiency improvements of 1.0 p.p. for pure convective cooling and 0.5 p.p. for combined convective and film with TBC are observed for Joule cycles. But just up to 0.5 p.p. for pure convective cooling and 0.3 p.p. for combined convective and film cooling with TBC are recorded for PGC cycles.

Overview and analysis of the experimentally measured throat heat transfer in liquid rocket engine thrust chambers

The design of high-performance liquid rocket engines is highly dependent on the management of the heat transfer rate produced by the combustion-gas and entering the thrust chamber wall. In particular, the life of the thrust chamber is mainly limited by the level of temperature and heat flux reached in the most thermally solicited part, which is the throat region. Consequently, experimental data of the hot-gas side heat transfer have been acquired since the 1950s. Such studies have increased over time in order to investigate the effect of different propellant combinations, propellant injector configurations, thrust chamber geometries, and combustion chamber pressures. In the present study, about 500 experimental heat transfer data taken from the open literature and relevant to the throat region are collected, manipulated, and analyzed with the main aim to find simplified rules to evaluate the peak heat transfer. Due to the intrinsic variability of the hot-gas side heat transfer and to the non-negligible measurement errors, the found correlations fit the whole set of experimental data with an uncertainty of up to 60%. However, considering specific propellant combinations, like oxygen–hydrogen or oxygen–hydrocarbons, far more reliable regressions are found.

Different tube bundles effect on the shell-and-tube heat exchanger performance

Purpose This paper aims to present a numerical investigation on laboratory-scale segmental baffles shell-and-tube heat exchanger (STHX) having various tube bundles and baffle configuration. Design/methodology/approach To discover the higher performance the thermohydraulic behavior of shell-side fluid flow with circular, elliptical and twisted oval tube bundles with segmental and inclined segmental baffled is compared. Shell side turbulent flow and heat transfer are simulated by a finite volume discretization approach using SolidWorks Flow Simulation. To achieve greater configuration performance of this device, the following two approaches is considered: using the inclined baffle with 200 angles of inclination and applying the different tube bundle. Findings Different parameters as heat transfer rate, pressure drop (Δp), heat transfer coefficient (h) and heat transfer coefficient to pressure drop ratio (h/Δp) are presented and discussed. Besides, for considering the effect of pressure penalty and heat transfer improvement instantaneously, the efficiency evaluation coefficient (EEC) in the fluid flow and heat transfer based on the power required to provide the real heat transfer augmentation are used. Originality/value Obtained results displayed that, at the equal mass flow rate, the twisted oval tubes with segmental baffle decrease the pressure drop 53.6% and 35.64% rather than that the circular and elliptical tubes bundle, respectively. By comparing the (h/Δp) ratio, it can result that the STHX with twisted oval tubes bundle (both segmental and inclined baffle) has better performance than other kinds of the tube bundles. Present results showed that the values of the EEC for all provided models are higher than 1, except for elliptical tube bundles with segmental baffles. The STHX with twisted oval tube bundles and segmental baffle gives the highest EEC value equal to 1.16 in the range of investigated mass flow.

Development of a staged particle heat exchanger for particle thermal energy storage systems

A new concept of a staged moving bed particle heat exchanger comprising flow constrictions in multiple short stages is presented. This concept allows for a simpler design, and potentially improves particle side heat transfer as found in heat transfer modelling and experiment results. Staging the particle heat exchanger into a series of short sections could prevent the full thermal development of granular flow. The incorporated flow constriction arrangement also allows renewal of the thermal entrance zone and re-mixing of particles at each stage, thus enhancing the particle-side heat transfer coefficient.

Effect of Entrance Geometry on Heat Transfer in a Rib-Roughened Rectangular Channel

The effect of the entrance geometry conditions on the local and average heat transfer coefficient in a rectangular channel with 45° angled ribs has been investigated. Ribs were deployed on one side only, the remaining walls of the channel being assumed smooth and unheated. The influence of entrance conditions was addressed by considering either a long ribbed unheated section or a smooth channel upstream of the ribbed heated section. As known, channels with 45° angled ribs are characterized by spanwise variations of heat transfer coefficient due to secondary flows, with high heat transfer levels close to one side and low heat transfer levels on the opposite side. Numerical results, checked against experimental data, showed that heat transfer coefficients of first modules are affected by entrance conditions, especially in the most effective heat transfer region. Regardless of entrance condition, a quasi-periodically fully developed heat transfer condition is attained only in the low heat transfer region, while elsewhere heat transfer coefficient distributions are periodic in shape but values tend to increase with the streamwise coordinate. These findings indicate that heat transfer experiments on ribbed channels should be generally accompanied by a careful description of the entrance conditions assumed in the experiments.