Heat and Mass Transfer最新文献

Turbo-molecular pumps are widely used in industries and scientific studies in areas such as semiconductors and sedimentation. In; such pumps, the influence of the geometric characteristics and operating conditions on performance is of interest to researchers. A single-row rotor of a turbo-molecular pump was simulated, and the test particle Monte Carlo method was used to investigate the effects of blade spacing, angle, number, and length; as well as operating conditions on performance. The maximum compression ratio and pumping speed were calculated. The pump’s performance was studied for 12, 24, 36, and 48 blades in a single-row rotor at angles of 15, 20, and 30 degrees with a clearance of 0.015. We were increasing the gap from 0.015 to 0.025 and 0.035 examined across rotors with 12 to 48 blades, a blade speed ratio of 3.5, and a blade angle of 15 degrees. To validate the results, the numerical values obtained for 36 blades, 30-degree blade angle, and 0.015 clearance were compared to available experimental results, yielding good agreement. The results of this simulation are intended to cover cases of interest for design calculations.

This work conducts a numerical investigation of water jet impingement cooling during the steel quenching process. Although much of the simulation work in the literature relies on two-dimensional analyses, this study developed a three-dimensional CFD simulation model using Ansys-Fluent. The Eulerian mixture formulation with the volume of fluid (VOF) method was employed. It is shown that the developed model accurately simulates the boiling behavior in impingement cooling using circular water jets. The main parameters used in the simulation were: initial surface temperature of 700 °C, and jet velocity of (0.4,m/s) impinging from a nozzle at (8,mm) height from the heated surface. The 3D mesh has been refined in a way to maintain a ({y}^{+}) value of 1 at the heated surface to capture the physics on the surface and to ensure that the viscous boundary layer is captured. Results such as temperature drop, boiling curve, and bubble frequency were presented and verified with the available experimental work in the literature. The developed mixture simulation using Ansys-Fluent has demonstrated its capability to numerically simulate the temperature history and boiling curves in the impingement process. This advancement will facilitate the study of numerous industrial parameters that are challenging to investigate experimentally.

Abstract

This work proposes a novel flat heat pipe technology, namely the rod-plate heat pipe, formed by the diffusion bonding of a set of parallel rods, of around 8 mm diameter, between flat plates of approximately 500 × 60 × 2 mm³. This design is inspired by the mini wire-plate heat pipe concept. This work is the first in the literature to apply this technology to large size heat pipes. A theoretical model is devised and used to predict the fluid distribution along the heat pipe, detect regions of flooding and dry-out and determine the best charging volume. Experiments are performed with a stainless-steel device operating in horizontal orientation with water as working fluid. Electrical cartridge resistances play the role of the evaporator heat source, while the condenser is cooled by either natural convection and radiation or heat exchangers linked to a thermal bath. For the experiments using a device with an exposed condenser, the minimum thermal resistance is 0.147 °C/W, for 88.50 W for heat input. The operation temperature increases with heat input up to 326.56 °C for a heat load of 191.40 W. The thermal resistances of the heat pipe cooled by heat exchangers have a minimum of 0.123 °C/W at 171.57 W heat transport rate, for a 40 °C thermal bath temperature. The theoretical results and data obtained so far corroborate the feasibility of this technology, with devices able to transfer up to 22.18 W per groove.

This study presents the pool boiling heat transfer performance of R-513A, a low-GWP alternative to R-134a, on a smooth tube at various saturation temperatures. The tests were carried out in the heat flux range of 10 to 90 kW/m². The experimental results show that at low heat flux (q^{primeprime}) ≤ 30 kW/m²_, the heat transfer coefficient (HTC) of R-513A is equivalent to R-134a, while at heat flux 30 ≥ (q^{primeprime}) ≥ 90 kW/m², the average HTC of R-513A is nearly 14% lower than that of R-134a irrespective of all the saturation temperatures. While the average HTC of pure R-513A is nearly 13% higher than that of R-1234ze(E). Further, the HTC of R-513A is compared with other similar alternative low GWP refrigerants R-1234yf, R-1234ze(E), and R-450A. According to the literature review, R-513A and R-1234yf perform almost equally well in terms of heat transfer during pool boiling. Depending on the boiling surface and testing setup, R-1234yf's pool boiling heat transfer performance is either less than or equal to R-134a.

The present study elucidates the experimental investigations on a novel flexible heat transfer device that can be used in a wide range of modern electronic device cooling applications which demand flexibility. The objective of the present is to address the challenges encountered by current flexible heat transfer devices, including concerns related to out-gassing and the permeation of non-condensable gases. These issues ultimately contribute to the deterioration of the long-term dependability of such devices. The present study provides an analysis of the steady-state performance of the flexible heat transfer device under various heat loads and orientations (0°, 45°, and 90° angles). Using COMSOL Multiphysics 6.1, numerical simulations are performed to explain the dynamics of heat transfer of the flexible heat transfer device developed. The performance is evaluated in terms of thermal resistance, equivalent thermal conductivity, and average temperature difference across the evaporator and condenser. Under steady-state operation, it has been determined that the flexible heat transfer device exhibits a minimum thermal resistance of 2.3 °C/W. Additionally, a maximum effective thermal conductivity of 2407 W/mK has been reported for a bending angle of 45°, which is six times more than relevant flexible heat transfer devices, such as copper thermal straps.

The rapid fluctuations in heat transfer rates make it challenging to determine the surface temperature history and the estimation of accurate heat generation in research applications such as IC engines, gas turbines, and high-speed space vehicles. Therefore, thin-film heat flux sensors (TFHFS) are generally used to measure the heat flux in such applications due to their high sensitivity and quick response time. The present study demonstrates that increasing the annealing heat treatment temperature will enhance the adhesion of the thin film and the capabilities of these hand-made TFHFS for transient measurements at low temperatures and for short periods. In the present work, TFHFS is fabricated in-house using platinum as a sensing element and Macor as an insulating substrate. The sensitivity (S) and temperature coefficient of resistance (TCR) are estimated using an oil batch calibration technique. At the same time, the performance of TFHFS is tested in a dynamic convective environment. The TFHFS is exposed to the convective environment using a designed calibration set-up, and their transient heat fluxes are computed by conducting several trials. Additionally, the numerical solution has been accomplished using various experimental parameters. In comparison to the outcomes of the experimental method, it is observed that the average fluctuating temperature and mean surface heat flux have an inaccuracy of 0.33% and 4.17% respectively.

Calculating a two-phase pressure drop is required in many fields. A general and accurate correlation is still needed, although numerous studies of frictional pressure drop correlations have been conducted. Therefore, a wide range of experimental data points were collected on the two-phase pressure drop of smooth tubes, 2012 points, covering 16 refrigerants with tube diameters ranging from 1.88 to 12 mm. The experimental data points were compared and evaluated with nine widely used correlations and models. The results show that the (Kim and Mudawar in Journal of Int J Heat Mass Transf 55:3246-3261, 2012) correlation gives the best prediction, followed by the Wang et al. (Exp Therm Fluid Sci 15:395–405, 1997) correlation and Friedel (European Two-Phase Flow Group Meeting, Ispra, Italy, 1979) correlation with mean deviations of 26.94%, 27.26% and 29.76%, respectively. A general two-phase frictional pressure drop correlation is proposed. The proposed correlation is validated against the database and compared with other correlations. The proposed correlation agrees better with the database than the others, with an absolute mean deviation of 19.84%.

Efficient heat transfer technologies are critical in a wide range of industrial applications, including air conditioning, chemical reactors, and heat exchangers. One method for improving heat transfer performance is to use twisted tape inserts in heat exchanger tubes. Heat transmission is aided by the disturbance of fluid flow caused by these inserts, although research is still ongoing to establish the specific design components that maximize their efficacy. The research focuses on heat transfer optimization in practical applications by exploring hexagonal perforated twisted tape inserts with varied cut orientations (horizontal, vertical, and alternate) and a pitch ratio of 4. The problem becomes more complex without a complete numerical prediction model. The study seeks to construct a hybrid deep neural network based on a gannet optimization algorithm (DNN-GOA) model in order to estimate heat transfer performance accurately. According to the experimental results, the TTA’s specific design with alternate cuts produces a thinner thermal boundary layer and a higher convective heat transfer coefficient for Nusselt number (Nu), friction factor (f), and thermal performance factor (TPF). The Hybrid DNN-GOA model has the best predictive performance, with a high R² indicating a tight match between anticipated and real Nu, f, and TPF values. It also exhibits the lowest Root Mean Square Error (RMSE), Mean Absolute Error (MAE), Mean Absolute Percentage Error (MAPE), and Mean Squared Error (MSE), confirming its exceptional accuracy.

Abstract

In this experimental study, we explore the potential enhancements in thermal conductivity while investigating alterations in latent heat and phase change temperature within Composite Phase Change Materials (PCMs). These composites consist of Paraffin Wax (PW) as the base material, incorporating dispersed conducting Polyaniline (PANI) powder in varying concentrations ranging from 1% wt. to 4% wt. The mass fractions of PANI added to PW include 1%, 2%, 3%, and 4%, and the composite PCMs are meticulously prepared through ultrasonication. Examining the surface morphology of Composite Phase Change Materials (PCMs) involved utilizing a Scanning Electron Microscope (SEM), while the determination of thermal conductivity employed a Heat Flow Meter. Additionally, latent heat and phase change temperatures were assessed through Differential Scanning Calorimetry (DSC). The obtained results indicate an augmentation in the thermal conductivity of the composites when compared to Paraffin Wax (PW). Specifically, thermal conductivity exhibited a 40% increase for 1% wt. of PANI, yet experienced a subsequent decline for the remaining weight percentages. Furthermore, the latent heat and phase change temperatures of the composites were observed to decrease in comparison to PW. These composite PCMs with enhanced thermal conductivity, achieved through the incorporation of Polyaniline in Paraffin Wax, are highly potential for several applications in energy storage systems, thermal regulation devices, and heat management technologies.

The energy crisis we are currently experiencing is merely the start of a very challenging and wide transformation. The sectors of power, coal, and natural gas encountered the biggest vibrations. To help with energy conservation, a compact and effective heat exchanger was made available that may be utilized to collect waste heat from power plants and industries. This study investigates the effects of combining passive techniques on the performance of a double-pipe heat exchanger equipped with a metal turbulator on the hot side and Al₂O₃ nanofluid on the cold side. The experiments used different volume fractions of Al₂O₃ nanofluid (Vol.%: 0.05, 0.1, and 0.15) as cold fluid with varying flow rates (500 ≤ Re ≤ 5000) in the annulus, as well as variously configured twisted tapes (Twist ratio: 20, 13.3, and 9.8) and frequently spaced helical screw tapes (Number of helices: 5, 7 and 9). The results show that the Nusselt number increases by 11.11% and the thermal performance factor increases by 1.116 times in case of twisted tapes with twist ratio 20 and 0.05% nanofluid combination, and by 24.93% and 1.269 times in case of frequently spaced helical screw tape with 9 number of helices and 0.15% nanofluid combination, respectively. Therefore, even at the expense of a small amount of pressure loss, 9 helices with 0.15% of Al₂O₃ nanofluid offered better performance in the combinations evaluated.