Fixed Heat Load Research Articles

Optimising design and operation of sewage source heat pump: techno-economic and environmental assessment

ABSTRACT Heat pump can be used to recover abundant thermal energy contained in the discharge of municipal wastewater treatment plants. While there are some design standards for common heat pump systems, the design of a sewage source heat pump (SSHP) system is still often based on a fixed heat load and neglects the interdependencies between the equipment sizing and operating parameters. To address the issue that previous design methods have not balanced investment and operational costs well from a global optimisation perspective, this work formulates the simultaneous optimisation of SSHP design and operation as a non-linear programming problem. The proposed model features the consideration of multiple working conditions caused by the impact of ambient temperature variation on the heat load of the SSHP system. The feasibility and potential benefits of the optimised SSHP system are also evaluated by incorporating techno-economic performances and environmental impact analyses into the mathematical framework. A case study is carried out to demonstrate the effectiveness of the proposed methodology. The results show that the total annual cost of the optimally designed and operated SSHP in Harbin could be 9% lower than in Beijing and 39% lower than in Shanghai, suggesting that constructing and running the SSHP system in severe cold regions with great heating demands might be more economical than in less cold regions. The CO2, SO2, and NOx emissions of the SSHP could be approximately 50% less than that of coal-fired boiler heating, and 80% less than that of direct electric heating with coal-fired electricity.

THERMO-HYDRAULIC OPTIMIZATION OF PLATE HEAT EXCHANGER IN ORC FOR A RATED HEAT LOAD

The efficiency of an Organic Rankine Cycle which utilizes low-grade waste heat or heat from renewables like solar depends on the performance of its condenser infinite surface area, the infinitesimal temperature difference is needed for effective heat transfer. Practically, neither are possible in a heat exchanger and therefore designers tend to improve effectiveness at a finite temperature difference of the two fluid streams and finite areas that are practically possible, based on other design constraints like space. The present work aims to optimize geometrical parameters of a plate heat exchanger that meets fixed heat load so as to minimize the area of heat transfer and pressure drop. An evolutionary genetic algorithm is used with a single objective function combining the dual objectives area and pressure drop by giving appropriate weights to these functions. Optimization is carried out to find optimal geometrical parameters width, length, the distance between plates, and enlargement factor for minimizing the area and also the pressure drop. Also, different weights are given to the two objective functions to compare optimal design variables for varying requirements of objective functions and optimal values of geometrical parameters width, length, the distance between plates and enlargement factor are determined.

Transient Thermal Behavior of a Neon Pulsating Heat Pipe (PHP)

Pulsating heat pipes (PHPs) are more and more studied as thermal link to cool down superconducting magnet using cryocoolers. They are passive two-phase heat transfer devices where the condenser part is connected to the evaporator part by a long capillary tube bent into many U-turns. For years, only small pulsating heat pipe (less than 300 mm long) have been studied at cryogenic temperature, using N $_2$ , He, Ne, and H $_2$ as working fluids. We have developed a meter-long horizontal PHP for future superconducting magnets that would be effective both on earth or in space. All the experiments have been performed using neon as working fluid with a fixed heat load on the evaporator part. PHP with neon operates at a temperature around 30 K which makes these devices interesting for cooling down future HTS magnet. This paper presents the results and analyses of the PHP thermal behavior after a sudden high increasing of the heat load on the evaporator part. These information are necessary to foresee the thermal behavior and operating limits of such a device during the quench of a superconducting magnet. This paper shows that neon PHP can function normally with an important supplementary heat generated on the evaporator part.

Study on energy-saving reconstruction plans for cold end system of direct air-cooling unit

Taking several recent successful technological transformation cases as technical reference, technical transformation plans are purposefully reviewed, pointing at questions at high temperature, for example, high back-pressure and high coal consumption for power generation. The peak cooling system distributes part of steam turbine exhaust and the air-cooling condenser thermal load reduces with the fixed cooling capacity of air cooling system. Or the capacity-increasing improves the cooling capacity with the fixed heat load of cold end system. The chosen of technical plan of direct air-cooling unit should be made on the base of its real situation.

Estimation of performance of a J-T refrigerators operating with nitrogen-hydrocarbon mixtures and a coiled tubes-in-tube heat exchanger

The exergy efficiency of Joule-Thomson (J-T) refrigerators operating with mixtures (MRC systems) strongly depends on the choice of refrigerant mixture and the performance of the heat exchanger used. Helically coiled, multiple tubes-in-tube heat exchangers with an effectiveness of over 96% are widely used in these types of systems. All the current studies focus only on the different heat transfer correlations and the uncertainty in predicting performance of the heat exchanger alone. The main focus of this work is to estimate the uncertainty in cooling capacity when the homogenous model is used by comparing the theoretical and experimental studies. The comparisons have been extended to some two-phase models present in the literature as well.Experiments have been carried out on a J-T refrigerator at a fixed heat load of 10 W with different nitrogen-hydrocarbon mixtures in the evaporator temperature range of 100–120 K. Different heat transfer models have been used to predict the temperature profiles as well as the cooling capacity of the refrigerator. The results show that the homogenous two-phase flow model is probably the most suitable model for rating the cooling capacity of a J-T refrigerator operating with nitrogen-hydrocarbon mixtures.

Optimization of a Thermoelectric Cooler for Time-Varying Heat Load and Sink Temperature

Thermoelectric coolers (TECs) are solid-state cooling devices that operate on the Seebeck effect. They can be used in electronic cooling applications as well as other refrigeration systems. Among the various factors that affect TEC performance within a system, it has been shown that the thermal conductance is an important parameter, which can also be easily altered during the design of a TEC to deliver optimal TEC performance for a given application. However, these studies have considered only a fixed heat load and heat sink temperature, whereas in many realistic applications these quantities can vary. A procedure has been developed for optimizing the thermal conductance of a TEC based on a typical operating cycle of time-varying heat load and sink temperature, while permitting constraints that ensure that one or more worst-case operating conditions can also be met. This procedure is valid for any arbitrary heat load and sink temperature functions; however, for illustrative purposes, a simple heat load function at fixed sink temperature (and a sink temperature function at fixed heat load) is used. The results show that the optimal conductance can strongly depend on the operating cycle, and the corresponding reduction in electrical input work (and corresponding increase in net coefficient of performance (COP)) can be significant.

Simulation of thermal processes in a flat evaporator of a copper–water loop heat pipe under uniform and concentrated heating

A 3D model has been developed for investigating heat and mass transfer in a flat evaporator of a copper–water loop heat pipe. It takes into account heat-transfer processes in the active zone, the barrier layer of the wick, the wall and the compensation chamber. The problem was solved by the finite difference method with the use of a nonuniform grid adapted to the configuration of the flat evaporator and its geometric peculiarities. Investigations have been carried out for understanding the effect of the heating zone size on heat distribution in the evaporator. The heating area was 9cm2 with a uniform heat supply and 1cm2 with a concentrated one. Numerical simulation has been performed for a heat load range from 20 to 1100W. Data have shown that a decrease in the heating area at a fixed heat load results in both increasing temperature on the evaporator wall under the heater and local wick draining in the active zone. The results of the model have been verified using results of experimental tests.

Design optimization of a loop heat pipe to cool a lithium ion battery onboard a military aircraft

The present paper proposes optimization procedures for loop heat pipe (LHP) designed to cool the lithium-ion battery for airborne high energy electric lasers (HEL) without power consumption. The LHP is more efficient than the air cooling device using bleed air. The battery temperature rising enlarges the permanent loss of its capacity and makes it unusable and unsafe. Cold speedy air around a flying aircraft becomes a good heat sink for dissipating the battery heat. The design objective is to reduce the weight of the LHP, considering the range of battery operating temperature and operational reliability. For numerical analysis, the total system of the LHP is analyzed through a thermohydraulic model and the heat transfer of the porous wick is predicted by the thin liquid film model. The thermal characteristics of the groove section are found by an analytical solution and a finite difference method. The design optimization is executed by using approximation models for the cases of (1) a fixed heat load, (2) a varied heat load without uncertainty consideration, and (3) a varied heat load with uncertainty consideration about the generated heat of the battery. As result of the optimization process, the weight of the LHP was reduced significantly. The optimized LHP for the varied heat load with uncertainty consideration case is most reliable and robust with slight increase of the weight.

Optimal paths for minimizing entransy dissipation during heat transfer processes with generalized radiative heat transfer law

A common of finite-time heat transfer processes between high- and low-temperature sides with generalized radiative heat transfer law [ q ∝ Δ( T n )] is studied in this paper. In general, the minimization of entropy generation in heat transfer processes is taken as the optimization objective. A new physical quantity, entransy, has been identified as a basis for optimizing heat transfer processes in terms of the analogy between heat and electrical conduction recently. Heat transfer analyses show that the entransy of an object describes its heat transfer ability, as the electrical energy in a capacitor describes its charge transfer ability. Entransy dissipation occurs during heat transfer processes, as a measure of the heat transfer irreversibility with the dissipation related thermal resistance. Under the condition of fixed heat load, the optimal configurations of hot and cold fluid temperatures for minimizing entransy dissipation are derived by using optimal control theory. The condition corresponding to the minimum entransy dissipation strategy with Newtonian heat transfer law ( n = 1) is that corresponding to a constant heat flux rate, while the condition corresponding to the minimum entransy dissipation strategy with the linear phenomenological heat transfer law ( n = −1) is that corresponding to a constant ratio of hot to cold fluid temperatures. Numerical examples for special cases with Newtonian, linear phenomenological and radiative heat transfer law ( n = 4) are provided, and the obtained results are also compared with the conventional strategies of constant heat flux rate and constant hot fluid (reservoir) temperature operations and optimal strategies for minimizing entropy generation. Moreover, the effects of heat load changes on the optimal hot and fluid temperature configurations are also analyzed.

Optimization for entransy dissipation minimization in heat exchanger

A common of two-fluid flow heat exchanger, in which the heat transfer between high- and low-temperature sides obeys Newton’s law [q∝Δ(T)], is studied in this paper. By taking entransy dissipation minimization as optimization objective, the optimum parameter distributions in the heat exchanger are derived by using optimal control theory under the condition of fixed heat load. The condition corresponding to the minimum entransy dissipation is that corresponding to a constant heat flux density. Three kinds of heat exchangers, including parallel flow, condensing flow and counter-flow, are considered, and the results show that only the counter-flow heat exchanger can realize the entransy dissipation minimization in the heat transfer process. The obtained results for entransy dissipation minimization are also compared with those obtained for entropy generation minimization by numerical examples.

Experimental and Numerical Study of a Stacked Microchannel Heat Sink for Liquid Cooling of Microelectronic Devices

One of the promising liquid cooling techniques for microelectronics is attaching a microchannel heat sink to, or directly fabricating microchannels on, the inactive side of the chip. A stacked microchannel heat sink integrates many layers of microchannels and manifold layers into one stack. Compared with single-layered microchannels, stacked microchannels provide larger flow passages, so that for a fixed heat load the required pressure drop is significantly reduced. Better temperature uniformity can be achieved by arranging counterflow in adjacent microchannel layers. The dedicated manifolds help to distribute coolant uniformly to microchannels. In the present work, a stacked microchannel heat sink is fabricated using silicon micromachining techniques. Thermal performance of the stacked microchannel heat sink is characterized through experimental measurements and numerical simulations. Effects of coolant flow direction, flow rate allocation among layers, and nonuniform heating are studied. Wall temperature profiles are measured using an array of nine platinum thin-film resistive temperature detectors deposited simultaneously with thin-film platinum heaters on the backside of the stacked structure. Excellent overall cooling performance (0.09°C∕Wcm2) for the stacked microchannel heat sink has been shown in the experiments. It has also been identified that over the tested flow rate range, counterflow arrangement provides better temperature uniformity, while parallel flow has the best performance in reducing the peak temperature. Conjugate heat transfer effects for stacked microchannels for different flow conditions are investigated through numerical simulations. Based on the results, some general design guidelines for stacked microchannel heat sinks are provided.

Enhanced Heat Transfer Characteristics of Viscous Liquid Flows in a Chevron Plate Heat Exchanger

Steady-state heat transfer and pressure drop data for single-phase viscous fluid flows (2 ≤ Re ≤ 400) in a single-pass U-type counterflow plate heat exchanger (PHE) with chevron plates are presented. With vegetable oil as test fluid (130 < Pr < 290), three different plate arrangements are employed: two symmetric (β = 30 deg/30 deg and 60 deg/60 deg) and one mixed (β = 30 deg/60 deg). The effects of chevron angle β, corrugation aspect ratio γ, and flow conditions (Re, Pr, μ/μw on Nu and f characteristics of the PHE are delineated. The results show a rather complex influence of plate surface corrugations on the enhanced thermal-hydraulic behavior. Relative to the performance of equivalent flat-plate packs, chevron plates sustain up to 2.9 times higher heat transfer rates on a fixed geometry and constant pumping power basis, and require up to 48 percent less surface area for the fixed heat load and pressure drop constraint.

Flow and Mass Transfer Performance in Short Pin‐FinChannels with Different Fin Shapes

The mass transfer (analogous to heat transfer) and pressure loss characteristics of staggered short pin‐fin arrays are investigated experimentally in the range of Reynolds number 3000 to 18,000 based on fin diameter and mean approach‐flow velocity. Three different shapes of fins with aspect ratio of 2 are examined: one uniform‐diameter circular fin (UDCF) and two stepped‐diameter circular fins (SDCF1 and SDCF2). Flow visualization using oil‐lampblack reveals complex flow characteristics associated with the repeated production of horseshoe vortices and fin wakes, and the interactions among these. The SDCF1 and SDCF2 arrays show flow characteristics different from the UDCF array due to downflow from the steps. For all arrays tested, the near‐endwall flow varies row by row in the initial rows until it reaches a stable pattern after the third row. The row‐averaged Sherwood numbers obtained from the naphthalene sublimation experiment also show a row‐by‐row variation pattern similar to the flow results. While the SDCF2 array has the highest mass transfer rate, the SDCF1 array has the smallest pressure loss at the same approach‐flow velocity. The fin surfaces have higher array‐averaged Sherwood number than the endwall and the ratio between these changes with fin shape and Reynolds number. The performance of the pin‐fin arrays is analyzed under two different constraints: the mass[heat transfer rate at fixed pumping power, and the mass/heat transfer area and pressure loss to fulfill fixed heat load at a fixed mass flow rate. In both cases, the SDCF2 array shows the best performance.

Experiments on In-line Pin Fin Arrays and Performance Comparisons with Staggered Arrays

Heat transfer and pressure drop experiments were performed for in-line pin fin arrays to obtain basic data to complement available information for staggered arrays. The experimental data were utilized as input to analyses aimed at establishing performance relationships between in-line and staggered arrays. In the experiments, mass transfer measurements via the naphthalene sublimation technique were employed to determine the row-by-row distribution of the heat (mass) transfer coefficient. Fully developed conditions prevailed for the fourth row and beyond. In general, the fully developed heat transfer coefficients for the in-line array are lower than those for the staggered array, but the pressure drop is also lower. The deviations between the two arrays increase with increasing fin height. With regard to performance, the in-line array transfers more heat than the staggered array under conditions of equal pumping power and equal heat transfer area. On the other hand, at a fixed heat load and fixed mass flow rate, the staggered array requires less heat transfer surface than the in-line array.

NATURAL CONVECTION HEAT TRANSFER PERFORMANCE EVALUATIONS FOR DISCRETE-(IN-LINE OR STAGGERED) AND CONTINUOUS-PLATE ARRAYS

Basic heat transfer results for natural convection in an array of vertical in-line plate segments were obtained by numerical finite-difference solutions and are tabulated along with previously computed results for staggered and continuous-plate arrays. These results were employed as the basis of three performance comparisons Involving alt three types of arrays, with the continuous-plate array serving as a baseline case. In the first of these comparisons, it was found that the discrete-plate arrays can enhance the heat transfer rate by as much as 80-90% for fixed values of the wall-to-ambient temperature difference and heat transfer surface area. Furthermore, the use of discrete plates affords the possibility of reducing the wall-to-bulk temperature differences by as much as 35-40% at a fixed heat load and surface area. Reductions in the height of the array of up to 50% can also be achieved for conditions of fixed heat load and fixed wall-to-bulk temperature difference. The attainment of enhanced heat tran...