Numerical studies for the thermal regime of a high-speed railway tunnel considering piston action on seasonally frozen regions

piston action describes the phenomenon that air at the train nose is pushed forward by the increased pressure and air at the train rear is drawn forward by the decreased pressure when a train passes through a tunnel. The changes of pressure can affect the thermal environment inside the tunnel, and further cause frost damage. In this paper, a fluid-thermal-solid coupled numerical model considering piston action is developed. A high-speed railway tunnel in the northeast of China is taken as an example to explore the temperature distribution laws with computational fluid dynamic (CFD). Afterwards, the effects of air temperature and train velocity on temperature distribution are analyzed. The results show that the piston action can enhance the heat transfer between cold air outside the tunnel and tunnel structure, and can cause more serious frost damage especially at the entrance and exit. The temperature distribution is characterized by three zones, including disturbed zones at two sides of tunnel and undisturbed zone at tunnel middle. The freezing length is closely related to air temperature and train velocity. And also, the lengths are different at vault and rail of tunnel portal, which indicates that the anti-freezing measure should be different at these positions considering the cost. This paper can provide some reference for determining the anti-freezing fortified length of tunnels in cold regions.

TRANSIENT OPERATION OF SENSIBLE FIXED-BED REGENERATORS

Fixed-bed regenerator is a type of air-to-air energy exchanger and recently introduced for energy recovery application in HVAC systems because of their high heat transfer effectiveness. Testing of FBRs is essential for performance evaluation and product development. ASHRAE and CSA recently included guidelines for testing of FBRs in their respective test standards. The experiments on FBRs are challenging as they never attain a steady state condition, rather undergoes a quasi-steady state operation. Before reaching the quasi-steady state, FBRs undergo several transient cycles. Hence, the test standards recommend getting measurements after one hour of operation, assuming FBR attains the quasi-steady state regardless of test conditions. However, the exact duration of the initial transient cycles is unknown and not yet studied so far. Hence, in this paper, the duration of FBR's transient operation is investigated for a wide range of design and operating conditions. The test standards' recommendation for the transient duration is also verified. The major contributions of this paper are (i) quantifying the effect of design parameters (NTUo and Cr*) on the duration of transient operation and (ii) investigation of the effect of sensor time constant on the transient temperature measurements. The results will be useful to predict and understand the transient behavior of FBRs accurately.

The effect of single-sided ribs on heat transfer and pressure drop within a trailing edge internal channel of a gas turbine blade

Convective cooling in a gas turbine blade internal trailing edge channel is often insufficient at the sharp trailing edge. This study examines convective heat transfer and pressure drop within a simplified trailing edge channel. The internal passage has been modeled as a right triangular channel with a 9° angle sharp corner. Smooth baseline and ribbed copper plates were heated from underneath via a uniform heat flux heater and examined via infrared thermography. Non-uniformity in the heat flux due to conduction is corrected by a RANS conjugate heat transfer calculation, which was validated by the mean velocity, friction factor, and temperature fields from experiments and LES simulations. Nusselt number distributions illustrate that surface heat transfer is increased considerably with ribs, and coupled with the vortices in the flow. Heat transfer at the sharp corner is increased by more than twofold due to ribs placed at the center of the channel, due to secondary flow. The present partially ribbed channel utilizes secondary flow toward the corner, and is presumed to have better thermal performance than a fully ribbed channel. Thus, it is important to set the appropriate rib length within the channel.

Aerothermal Characteristics of Surface-Mounted Round-Edged Slit Ribs with Performance Optimization using Response Surface Methodology

The author has investigated the aerothermal characteristics of round-edged ribs with a continuous slit. The experiments have been performed by mounting an array of ribs on the bottom wall inside a rectangular duct. Heat transfer characteristics have been measured using Liquid crystal Thermography (LCT), whereas flow characteristics have been measured using two-dimensional particle image velocimetry (2D-PIV) technique. Experiments have been performed for flow over a rib having ~20% blockage ratio and 10% open area ratio. Geometrical parameters considered for the study are slit angle (a) and rib pitch to height ratio (p/e). Experiments have been performed for three distinct rib configurations having a values, i.e., 0°, 5° and 10° with different arrangements having p/e values of 5, 10 and 15, at four Reynolds number ranges from 6200-12200. The heat transfer results are evaluated by examining the surface and span wise-averaged distribution of augmented Nusselt Number. Flow field results are explained within the inter rib region by examining the time-averaged normalized velocity fields, streamlines, fluctuation statistics and vorticity distribution. Further, the impact of geometrical design parameters (a and p/e) on different performance parameters, i.e., overall averaged augmented Nusselt Number, Friction Factor Ratio and Thermal Performance Factor have been analyzed at all four Reynolds number using Response Surface Methodology (RSM). Finally, the desired correlations for the performance parameters have been documented, and found in accord with an uncertainty range of ±10%.

Nanofluid flow of Alumina-Copper/Water through isotropic porous arrays of periodic square cylinders: mixed convection and competent array shape

The flow of hybrid Alumina-Copper/Water nanofluid with mixed convection heat transfer from multiple square cylinders arranged in three different types of arrays, namely equilateral triangle (ET), rotated square (RS), and rotated rhombus (RR) in a heat exchanger has never been studied before the present study. Navier-Stokes and energy equations with a periodic condition in transverse direction for three array types having the same porosity are solved with finite volume methodology. The combined effect of aiding buoyancy (Richardson number Ri 0-2), configuration of square cylinders, and hybrid nanoparticle volume fraction (0-0.06) on flow dynamics and their impact on the overall heat transfer phenomenon through three different array configurations is thoroughly elucidated. The arrays' overall drag and friction coefficient increases with an increase in the strength of aiding buoyancy and nanoparticle volume fraction. An increment in Ri, and nanoparticle volume fraction, causes thermal boundary layer thinning and results in higher heat transfer rates across three arrays. With an increase in Ri from 0 to 2 at a nanoparticle volume fraction of 0.06, mean Nusselt number of ET, RS and RR arrays is increased by 161%, 5% and 32% respectively. While, with an increase in nanoparticle volume fraction from 0 to 0.06 at Ri=2, mean Nusselt number of ET, RS and RR arrays is augmented by 17%, 6% and 9% respectively. Finally, the efficient array configuration in terms of fluid-thermal behavior is proposed to design various heat exchange systems under differing operating conditions.

Coupled Impression of Radiative Thermal Flux and Lorentz Force on the Water Carrying Composite Nanoliquid Streaming Past an Elastic Sheet

Significance of the study: Hybrid nanofluids attract the attention of many current researchers due to the enhanced heat transport rate in many engineering and industrial applications. The influence of an inclined magnetic field over an exponentially stretched sheet in the presence of thermal radiation cannot be ignored and the literature available in this domain is scanty. The novelty of this communication is to explore the impact of inclined magnetic field and thermal radiative heat on the hybrid nanofluid consisting of and nanoparticles in the base fluid, water. Aim of the study: A mathematical model for hybrid nanofluid is proposed to study the influence of oblique magnetic field and thermal radiation on an exponentially elongated sheet. A comparision of the thermal characteristics of the hybrid nanofluid and the mono nanofluids is made. Research methodology: The governing flow equations are transformed into a system of ODEs with the assistance of similarity variables and are then computationally addressed using bvp4c.The graphs are displayed for velocity, heat measure and reduced frictional coefficients for selected flow parameters. Results: Hybrid nanofluid has 1-4 % growth in the rate of heat transfer when compared to mono nanofluid while it is 1-4.5% in comparison to viscous fluid for increasing radiation parameter. Conclusion: The outcomes of this work revealed that the heat transfer as a consequence of the dispersion of dual nanomaterials is more promising than the mono nanofluid. To accomplish very effective cooling/ heating in industrial and engineering applications, hybrid nanofluids can substitute mono nanofluids.

Influence of Drainage and Tilt on Heat Transfer Performance of Array Pulsating Cold End Heat Pipe

A new type of array pulsating cold section heat pipe was proposed, which consists of a T-shaped hot section and an array pulsating cold section. The special structure is available to drain the hot section of the heat pipe, and the installation method of the cold section has an important influence on heat transfer. For this reason, a detailed experimental study of heat transfer performance was carried out in this paper. It was found in the study that a capillary lifting force exists at the outlet of the cold section channel, which prevents the condensate from returning to the hot section, therefore, the hot section has to be drained; the drainage methods are divided into hot-section liquid drainage and hot-section capillary drainage, the latter is significantly better than the former; appropriate increase of the filling rate can improve the drainage effect of liquid drainage. The new heat pipe can adopt two methods, i.e., inclined cold section and vertical cold section. The reflux and heat transfer performance of the inclined cold section outperforms that of the vertical cold section, but the difference between the two methods gradually decreases with the increase of power. Under the same working conditions, the average temperature of the heat source of the new stainless steel heat pipe with the capillary drainage vertical cold section is lower than that of the aluminum fin radiator by 5.79%-10.78%, and the decreasing amplitude increases with the increase of the heating power.

Transport phenomena and evolution mechanism of the melt pool during a laser based metal melting process

This article presents a fully three-dimensional numerical study on the process of melt pool evolution. In order to overcome the simplifications used in many existing studies, an enthalpy method is developed for the phase change, and an accurate interface capturing method, i.e., the coupled volume-of-fluid and level set (VOSET) method, is employed to track the moving gas-liquid interface. Meanwhile, corresponding experimental studies are carried out for the purpose of validation. The obtained numerical results show the formed interface morphology during the process of melt pool with its typical sizes and are quantitatively consistent with those data measured in experiments. Based on the numerical results, the thermodynamic phenomena, induced by the interaction between heat and momentum exchange, occurring in the formation of melt pool are presented and discussed. Mechanisms of the melt pool evolution revealed in the present study provide a useful guidance for better controlling the process of additive manufacturing.

Experimental Investigation on Finned Solar Still with Enhanced Thermal Energy Storage

In the present study, a novel solar still incorporated with fins and phase change material (PCM) based energy storage, was designed. To investigate the influence of fins and energy storage unit, four cases of stills were considered. In case I, a conventional type was considered, whereas square hollow fins were fitted over the basin liner of the still in case II. In addition to fins as in case II, case III employs energy storage unit wherein PCM was packed beneath the basin liner. Case IV was similar to case III except the extension of fins into the storage unit. The addition of fins above the base liner improved the absorber surface area and the extension of the same beneath the basin liner enhanced the storage efficiency. Experiments were carried out on all the four modules with a constant basin water depth of 2 cm. The maximum productivity of the conventional solar still was found to be 3.25 litre/m2/day. On the other hand, the results reveal improvement in productivity of 17.54%, 48.61% and 55.69% with cases II, III and IV, respectively. Although stills with energy storage unit exhibited higher exergy efficiency, the presence of fins in the PCM increases the internal irreversibilities. The cost of water yielded by modified solar still (MSS) used in case IV is proved to be less as compared to conventional solar still (CSS). Further, the payback period of MSS is found to be lesser than that of CSS.

Performance Analysis of Channels in Adiabatic and Non-adiabatic Spiral Plate Heat Exchangers: A Thermodynamic Study

This paper presents a systematic analysis of the thermodynamic performance of spiral turns in spiral plate heat exchangers (SPHEs), with and without heat leakage to the environment. An optimal design algorithm for SPHEs is developed to find higher compactness and overall heat transfer coefficient by increasing channels' pressure drops, maintaining geometric aspect ratio and minimizing the total costs. To specify the rate of heat loss to the environment, rate of internal heat transfer and channel temperature distribution, a mathematical model is proposed based on mass and energy balance equations to model the SPHE as a hypothetical heat exchanger network (HENs). Entropy-based and entransy-based performance evaluation methods in Heat Exchangers (HEs) are also examined to investigate the impact of heat leakage on the performance and irreversibility of the SPHEs. A single-phase counter-current SPHE is then designed as a case study, to examine the proposed mathematical and performance assessment models. The case study is defined and analyzed based on heat leakage to the environment. Three scenarios are then introduced namely heat leakage and no heat leakage to the environment and transferring the net heat between the streams. Results highlight the applicability of the proposed mathematical modelling and temperature distributions of channels in thermodynamics analysis of SPHEs with/without heat leakage to the environment. The findings also suggest that smaller adiabatic SPHEs can be a suitable substitute for non-adiabatic SPHEs providing appropriate insulation that covers outermost channels and prevent the leakage of the heat to the environment.