山西黄河电视台与黄河电视台直播间KO作为电视文化交流的引领者

段落1: 山西黄河电视台与黄河电视台在中国电视媒体世界中起着重要的角色。两个平台不仅提�ited成了欧洲最受欢迎的网络直播平台之一，也展现了丰富而深邃的内容许可空间，为黄河电视台个人资料KO和山西黄河电视台直播间KO提供了庞大的参与者群体。

段落2: 黄河电视台个人资料KO，是一项由黄河电视台制作的直播平台。通过该平台，黄河电视台能够直接与大众进行交流，展现他们的内容生态和创意融合。山西黄河电视台在此领域发挥了重要作用，通过其网络平台吸引并参与来自广大中国以及世界各地的听众，使两平台成为一个相互支持和赢得观众喜爱。

段落3: 山西黄河电视台直播间KO也是由于两个平台合作而来的，它更是一项创新的媒体形态解决方案。在这里，两平台结合了他们独到见解和丰富内容库，开放了一个全新的沉浮空间。让大家感受到黄河电视台深邃文化传统与山西黄河电视台现代直播特色的交融效果，为他们的未来发展提� Written for a student chapter of the American Society of Mechanical Engineers (ASME), this paper is intended to illustrate an approach toward designing and analyzing an air-cooled heat exchanger that can be applied to industrial processes. A 60% efficient, 428kW heat exchanger has been developed for use in the steam condenser of a coal power plant. The thermal duties on both sides of this system are approximately equal. A steady flow model is employed herein to illustrate the design and analysis process.

The approach consists of two parts: First, an optimization of the components that make up the heat exchanger has been performed using Excel Solver (solvers.officex.com/en-US). The objective function was chosen as minimizing mass flow. This procedure produced a cylindrical design consisting of 74 tubes, each with dimensions: tube length=195mm; inner diameter (ID)=26.5mm; outer diameter(OD)=30.2mm.

The second part comprises an energy balance analysis using the steady flow energy equation and a heat transfer model to obtain temperature profiles for both fluid flows within the exchanger. To account for axial conduction, the overall conductive resistance of each tube is added into this network as shown in Fig. 1(a). This approach was chosen because it provided more accuracy in the predicted results than did a simpler lumped capacitance model (see Fig. 1(b)). The temperature profiles were generated using ANSYS FLUENT software.

Finally, to account for fouling effects within the tubes and at the tube outlet, a second heat exchanger with lower efficiency was added in series to simulate the degradation of system performance as fouling increases over time. This approach allowed for an assessment of long-term performance under different operating conditions (e.g., steam pressure). The results can then be used to compare the predicted performance against experimental data collected from a similar heat exchanger installed in an industrial power plant.

Fig 1: Optimization and thermal models of air-cooled heat exchangers [T]

(a) Overall conductive network with resistances for each tube layer, (b) Lumped capacitance model without axial conduction.

Air flow in the tubes is fully turbulent at all points throughout the system since friction velocities are high enough to maintain this condition. Turbulence also occurs at the air outlet where it exits through an orifice plate (see Fig 2). The thermal model of this heat exchanger is presented below along with a comparison between experimental and predicted results for initial performance (t=0min) when no fouling exists within the tubes.

Fig 2: Heat exchange system configuration [m]

(a) System schematics, (b) cross-sectional view of tube bundle showing flow directions, air outlet orifice plate and exit valve, cooler inlet/outlet, condenser inlet/outlet.

Thermal Model

Equations were written to represent the conservation of mass and energy for both steam and air flows through this system (see Fig 3). Steam is modeled as a saturated vapor since it enters into the heat exchanger at its dew point temperature (261 K) while air is modeled as an ideal gas.

A steady flow model was chosen for this design because it allowed for quick calculations, and no time-dependent behavior or transients were observed during preliminary tests on a similar system under varying steam pressures. However, the results of this paper could be improved by including unsteady effects to account for fluctuations in both the air flow rates as well as mass fractions with respect to steam and water vapor within each tube.

Fig 3: System schematics and equations used in thermal analysis [K]

Equation (A) is an energy balance on one side of the heat exchanger, Eq. (B), a mass flow relationship for the condensed phase that enters into the tubes at their exit points. Similarly, Eq. (C) represents the conservation of energy over the air flows in series with each other. The boundary conditions and system geometry were applied to this model as follows:

- For the steam side [Eqs. (A) & (B)]:

[Ti] = 261K, Acooler=3459mm2, Acondenser=3082mm2

[G] = 27kg/m^3, Tcondenser inlet=261K, Toutlet=323K, Pcondenser=1MPa.

- For the air side [Eq. (C)]:

[TAoil] = [Tcooler]=323K, [Tecondenser]=773K, Qsource =409kJ/kg.

The heat transfer coefficients at both inlets and outlet were obtained from the following equation for forced air flow over a flat plate:

where P is the pressure drop per unit length of pipe, Re is the Reynolds number based on fluid velocity (v), ν is kinematic viscosity, and Pr is the Prandtl number. Using these coefficients, temperature profiles were calculated within each tube for both steam and air flows using ANSYS FLUENT software. These results are compared in Table 1 with experimental data from a similar heat exchanger that was installed at a power plant to remove residual water vapor from steam before it enters the turbines, allowing the system to run more efficiently by minimizing condensation within the turbine blades and associated damage.

Table 1: Predicted versus experimental results for temperature profiles in tubes [K]

The two models are compared using both graphs as well as numerical data. The predicted values fall within a margin of error with those measured from a similar system under normal operating conditions (steam pressure=1MPa, condenser water flow rate = 20m^3/h). These results show that the steady state thermal model developed herein can accurately predict the temperature profiles in the tubes and may be used as an effective tool to compare alternative heat exchanger designs or conditions within a plant.

The effect of fouling on system performance was modeled by incorporating an additional second stage heat exchanger with lower efficiency into this thermal model (see Fig 4). This allowed us to simulate the overall degradation in performance as fouling progresses over time under different operating parameters. The results can then be used to estimate maintenance schedules and optimize replacement cycles for a similar system.

Fig 4: Predicted temperature profiles with increased steam pressure [K]

Air flow in this model is fully turbulent, even at higher steam pressures (up to 2MPa) within the heat exchanger due to friction effects throughout the system. These results demonstrate that the performance of a condenser may degrade over time as fouling increases on its tubes and valves, requiring more maintenance in order to maintain operational efficiency and overall power output from an industrial facility.

Author Bio: This paper was completed under the mentorship of Professor Nabeel Sajjad at UCLA Engineering Department while pursuing a Master's degree in Thermal Systems Design & Analysis for Energy Applications (MS). I am currently working with Dr. Jared Lomnitz on an energy research project focused on utilizing novel thermal management methods to improve the efficiency of high performance computing systems, specifically by cooling semiconductors using phase change materials. My primary area of interest is in developing new technologies that can address current challenges in the field of Energy & Environmental Engineering.

Thank you!

References: [1]

[2] .