• Title/Summary/Keyword: Energy Transfer

Search Result 4,180, Processing Time 0.038 seconds

A Study on Influence of Flow Boiling Heat Transfer on Fouling Phenomenon in Nanofluids (나노유체에서 파울링 현상이 유동 비등 열전달에 미치는 영향에 대한 연구)

  • Kim, Woojoong;Yang, Yongwoo;Kim, Younghun;Park, Sungseek;Kim, Namjin
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
    • /
    • v.28 no.3
    • /
    • pp.95-102
    • /
    • 2016
  • A boiling heat transfer is used in various industry such as power generation systems, heat exchangers, air-conditioning and refrigerations. In the boiling heat transfer system, the critical heat flux (CHF) is the important factor, and it indicated safety of the system. It has kept up studies on the CHF enhancement. Recently, it is reported the CHF enhancement, when working fluid used the nanofluid with high thermal properties. But it could be occurred nanoflouling phenomenon from nanoparticle deposition, when nanofluid applied the heat transfer system. And, it is reported that the safety and thermal efficiency of heat transfer system could decrease. Therefore, it is compared and analyzed to the CHF and the boiling heat transfer coefficient on effect of artificial nanofouling (coating) in oxidized multi-wall carbon nanotube nanofluids. As the result, the CHF of oxidized multi-wall carbon nanofluids and the CHF of artificial nanofouling in the nanofluids increased to maximum 99.2%, 120.88%, respectively. A boiling heat transfer coefficient in nanofluid increased to maximum 24.29% higher than purewater, but artificial nanofouling decreased to maximum -7.96%.

AIR ENTRAINMENT AND ENERGY DISSIPATION AT STEPPED DROP STRUCTURE

  • Kim Jin Hong
    • Water Engineering Research
    • /
    • v.5 no.4
    • /
    • pp.195-206
    • /
    • 2004
  • This paper deals with oxygen transfer by air entrainment and energy dissipations by flow characteristics at the stepped drop structure. Nappe flow occurred at low flow rates and for relatively large step height. Dominant flow features included an air pocket, a free-falling nappe impact and a subsequent hydraulic jump on the downstream step. Most energy was dissipated by nappe impact and in the downstream hydraulic jump. Skimming flow occurred at larger flow rates with formation of recirculating vortices between the main flow and the step comers. Oxygen transfer was found to be proportional to the flow velocity, the flow discharge, and the Froude number. It was more related to the flow discharge than to the Froude number. Energy dissipations in both cases of nappe flow and skimming flow were proportional to the step height and were inversely proportional to the overflow depth, and were not proportional to the step slope. The stepped drop structure was found to be efficient for water treatment associated with substantial air entrainment and for energy dissipation.

  • PDF

Assessment of Total Transfer Capability Based on Energy Function (에너지 함수를 이용한 총송전용량 평가)

  • Kim, Kyu-Ho;Kim, Soo-Nam;Rhee, Sang-Bong;Lee, Sang-Keun;Song, Kyung-Bin
    • Proceedings of the KIEE Conference
    • /
    • 2009.07a
    • /
    • pp.241_242
    • /
    • 2009
  • This paper presents a method to assess total transfer capability (TTC) by using energy function. To get the critical energy, the potential energy boundary surface(PEBS) method which is one of the transient energy function(TEF) method is used. TTC assessment is to calculate TTC by using the repeated power flow (RPF) method. It is seen that energy margin can be use to assess available transfer capability(ATC).

  • PDF

Effect of Natural Convection on the Heat Transfer in a Latent Heat Storage System (잠열축열시스템의 축열과정에서 자연대류의 영향에 관한 연구)

  • Ryu, S.N.;Han, G.Y.
    • Solar Energy
    • /
    • v.19 no.2
    • /
    • pp.29-36
    • /
    • 1999
  • Heat transfer characteristics of a low temperature latent heat storage system during the heat storage stage was examined for the circular finned tubes using fatty acid which shows the big density difference during melting as phase change materials. The heat storage vessel has the dimension of 530 mm height, 74 mm inside diameter and inner heat transfer tube is 480 mm in height and 13.5 mm outside diameter. Hot water was employed as the heat transfer fluid. During the heat storage stage, it was found that both conduction and natural convection were the major heat transfer mechanism. It was also found that the effect of natural convection on the heat transfer was more significant for the unfinned tube system than that for the finned tube system. The experimentally determined overall heat transfer coefficients were in the range of $50{\sim}250W/m^2K$ and the correlation for natural convection heat transfer as a function of Nusselt and Rayleigh number was proposed.

  • PDF

Analysis of the Efficiency According to Resonant Repeater Application in Magnetic Resonant Wireless Power Transfer System (자기공진방식의 무선전력전송 시스템에서 공진 중계기 적용 여부에 따른 전력전송 효율 분석)

  • Baek, Seung-Myung;Kim, Dong-Eun;Shon, Jin-Geun
    • The Transactions of the Korean Institute of Electrical Engineers P
    • /
    • v.67 no.4
    • /
    • pp.221-226
    • /
    • 2018
  • In this paper, the power transfer efficiency analysis based on the resonant repeater in a magnetic resonance wireless power transfer system is proposed. The efficiency of the magnetic resonance method was verified by comparing the general frequency with the resonance frequency. The resonance repeater was arranged to increase the efficiency and increase the transfer distance. When using resonant repeaters, the maximum efficiency increase is about 36.23[%] and the transfer distance was extended to more than 20[cm]. Through this study, confirmed the effect of using resonance repeaters in wireless power transfer system. As a result, it can be expected that the overall technology related to wireless power transfer system will be more valuable for energy-IT technology.

Heat Transfer Coefficients of Concentric Annuli for Testing Heat Transfer Characteristics of Alternative Refrigerants in Tubes (대체냉매 관내 열전달특성 시험을 위한 동심이중원관의 환상유로의 열전달계수)

  • KIM, MAN-HOE
    • Journal of Hydrogen and New Energy
    • /
    • v.32 no.1
    • /
    • pp.63-67
    • /
    • 2021
  • Accurate measurements of the heat transfer coefficients of concentric annular space for the test section is important to measure the tube-side heat transfer coefficients of working fluids. This paper presents the annular side heat transfer coefficients of concentric annuli with variation of tube diameter ratios using Wilson plot method. The test facility has a straight, horizontal test section with an active length of 3.0 m. Inner/outer diameters of test tubes are 7.0/7.5 and 8.0/8.56 mm, respectively. An outer diameter of annulus side is 16.0 mm. The test results show that convective heat transfer coefficients in annuli increase with annular diameter ratio. The correlations for convective heat transfer coefficients in annuli are also developed.

Experimental investigation on heat transfer of nitrogen flowing in a circular tube

  • Chenglong Wang;Yuliang Fang;Wenxi Tian;Guanghui Su;Suizheng Qiu
    • Nuclear Engineering and Technology
    • /
    • v.56 no.2
    • /
    • pp.463-471
    • /
    • 2024
  • Average and local convective heat transfer coefficients of nitrogen are measured experimentally in an electrically heated circular tube for a range of Reynolds number from 1.08 × 104 to 3.60 × 104, and wall-to-bulk temperature ratio from 1.01 to 1.77. The exit Mach number is up to 0.17, and the heat flux is up to 46 kW·m-2. The molybdenum test section has a 62 diameters heated section with an inside diameter of 5 mm and a 30 diameters entrance section to ensure the fully-developed flow. Uncertainty of Nusselt number is less than 1.6 % in this study. The results indicate that the average heat transfer correlations evaluated by both the bulk and the modified film Reynolds numbers agree well with the experimental data. The local heat transfer results based on bulk properties are compared with previous empirical correlations. New prediction correlations are recommended which are significantly affected by the property variation and heated length. The comparison between the proposed correlations and experimental points shows that 88 % of experimental data fall into an error of 10 %, and almost all data are within an error of 20 %.

Numerical Study of Heat Transfer Efficiency, Performace and Mechanical Behavior induced by Thermal Stress of Energy Pile (에너지 파일의 열교환 효율 및 성능, 열응력에 의한 역학적 거동 평가)

  • Min, Sun-Hong;Lee, Chul-Ho;Park, Moon-Seo;Koh, Hyung-Seon;Choi, Hang-Seok
    • Journal of the Korean Society for Geothermal and Hydrothermal Energy
    • /
    • v.6 no.2
    • /
    • pp.9-14
    • /
    • 2010
  • The ground source heat pump system is increasingly being considered as an alternative to traditional heating and cooling systems to reduce the emission of ground house gases. In this paper, A series of numerical analysis for energy piles has been performed focusing on heat transfer efficiency, performance and thermal stress. Results of numerical analyses for the W-shape type shows more efficient heat exchange transfer than the coil type. From results of the thermo-mechanical analysis, it is shown that the concentration of thermal stress occurs around the circulating pipe and the interfaces between different materials. The largest deformation caused by thermal stress is observed in the energy pile.

Load transfer and energy absorption in transversely compressed multi-walled carbon nanotubes

  • Chen, Xiaoming;Ke, Changhong
    • Coupled systems mechanics
    • /
    • v.6 no.3
    • /
    • pp.273-286
    • /
    • 2017
  • We present a simple and easy-to-implement lumped stiffness model to elucidate the load transfer mechanism among all individual tube shells and intertube van der Waals (vdW) interactions in transversely compressed multi-walled carbon nanotubes (CNTs). Our model essentially enables theoretical predictions to be made of the relevant transverse mechanical behaviors of multi-walled tubes based on the transverse stiffness properties of single-walled tubes. We demonstrate the validity and accuracy of our model and theoretical predictions through a quantitative study of the transverse deformability of double- and triple-walled CNTs by utilizing our recently reported nanomechanical measurement data. Using the lumped stiffness model, we further evaluate the contribution of each individual tube shell and intertube vdW interaction to the strain energy absorption in the whole tube. Our results show that the innermost tube shell absorbs more strain energy than any other individual tube shells and intertube vdW interactions. Nanotubes of smaller number of walls and outer diameters are found to possess higher strain energy absorption capacities on both a per-volume and a per-weight basis. The proposed model and findings on the load transfer and the energy absorption in multi-walled CNTs directly contribute to a better understanding of their structural and mechanical properties and applications, and are also useful to study the transverse mechanical properties of other one-dimensional tubular nanostructures (e.g., boron nitride nanotubes).

Numerical Investigation on Nonequilibrium Energy Transfer in Thin Metal Film Structures during the Irradiation of Femtosecond Pulse Laser (펨토초 레이저가 조사되는 동안의 금속 박막내의 비평형 에너지 전달 현상에 대한 수치해석 연구)

  • Sim, Hyung-Sub;Lee, Seong-Hyuk
    • The Transactions of The Korean Institute of Electrical Engineers
    • /
    • v.56 no.2
    • /
    • pp.367-373
    • /
    • 2007
  • The present study investigates numerically nonequilibrium energy transfer between electrons and phonons in metal thin films irradiated by ultrashort pulse lasers and it also provides the temporal and spatial variations of electron and phonon temperatures using the well-established two-temperature model(TTM) on the basis of the Boltzmann transport equation(BTE). This article predicts the crater shapes in gold film structures, and compares the results by using two-dimensional energy transport equation. From the results, it is found that nonequilibrium energy transfer between electrons and phonons takes place, and the equilibrium time increases with the increase of laser fluence. On the other hand, above threshold fluence the ablation time doesn't change nearly with increasing fluences. Compared with one-dimensional TTM, it also reveals that the temporal distributions of electron and phonon temperatures at the top surface estimated by using two-dimensional TTM have a similar tendency. The results show that two-dimensional TTM can simulate the crater shape of metals during the irradiation of femtosecond pulse lasers and the absorbed energy is propagated to z-direction faster than to r-direction.