Thermosiphon Flow Research Articles

Numerical Study of the Effects of a Hot Obstacle on Natural Convection Flow Regimes

A numerical study was conducted for natural turbulent and laminar convection induced by a circular hot obstacle in two different positions, the heat obstacle is introduced at the inlet of the cylinder and inside the cylinder. To determine the effects of a circular hot obstacle on the regime of the thermosiphon flow generated by a heated cylindrical channel, we suggest a theoretical and numerical model using the Navier–Stokes equations and the finite volume method. An experimental apparatus was used for the validation of the numerical model. A comparison of the numerical and experimental results showed a good agreement in the global flow behavior. The temperature and vertical velocity profiles demonstrated the presence of a boundary-layer regime along the heated channel's wall in the absence of a hot obstacle and the presence of the hot obstacle at different positions. The introduction of the hot obstacle generates a significant modification of the evolution of the flow above the hot obstacle. The numerical simulation was conducted for a specific radius of the hot obstacle that ensures the existence in the laminar flow regime, and different Rayleigh numbers (Ra=105; 107; 108; 109; and1010). The flow rate, velocity, temperature and Nusselt number profiles were presented and discussed. The effects of the hot obstacle are presented in our results by two maxima of the vertical component of the velocity profile. The temperature profile at the entrance of the vertical cylinder is presented by the appearance of a maximum above the hot obstacle. As the cylinder height increases, the air temperature decreases. The vertical displacement of the obstacle upward induces a modification in the behavior of the dynamical flow. A CFD code was used to study the different natural convection regimes presented by three different form factors, including the critical regime with a form factor of A=0.1.

DYNAMICAL EFFECTS OF A HOT DISK ON THE DEVELOPMENT OF A NATURAL CONVECTION FLOW BETWEEN CYLINDRICAL WALLS

This study numerically investigates the problem of natural convection flow ina vertical cylinder. The importance of this issue mainly stems from thermal systems (e.g. chimneys, hot air generators, solar collectors, and many others.). Despite the numerous studies on the modeling of free convection flow between two perpendicular plates, there are very few studies that have investigated regarding the convective flow between two cylindrical walls. Therefore, the study is limited to vertical cylinders with different heating modes.The first configuration includes heated walls of thermosiphon flow in a cylindrical channel.The second configuration introduces a circular heat source at the channel entrance, and the third configuration includes an ended vertical cylinder with a heat source centered in the cylinder (zs = 0.04). The flow comparison from the three configurations demonstrates how the hot disk affects the flow from a dynamic point of view.Furthermore, details about the flow and dynamical fields can be obtained from the solution equation of the conservation of momentum and energy, considering the differences between the boundary conditions of the studied configurations. The study covers Rayleigh numbers and focuses on the effect of the cylinders geometry on the characteristic of the flow as well as dynamical and thermal field characteristics and variations in Nusselt number. The studied flow is natural convection, laminar, steady, two-dimensional, and incompressible flow. Solutions were obtained using a numerical model based on the finite volume method and the Navier–Stokes equations. The velocity–pressure coupling was resolved using the SIMPLER algorithm. The temperature and vertical velocity profiles of the flow showed the existence of a boundary layer regime along the heated channel wall without a heat source. The layer continued along the cylindrical wall with the introduction of the heat source at the channel inlet. However, introducing the heat source caused a considerable change in the flow structure at the channels central part.Calculations were performed for the channel aspect ratio R* = 0.2 and different Rayleigh numbers (Ra = 105, 107, 108, 109 and 1010). Numerical results included velocity, temperature, and Nusselt number profiles.

Impact of thermosiphoning on long-term behavior of closed-loop deep geothermal systems for sustainable energy exploitation

Circulation of working fluid in closed geothermal loops is an alternative environmentally friendly approach to harvest subsurface energy compared to open hole geothermal doublet systems. However, the rapid decline of production temperature, low generated thermal power, and difficulties in deepening the system are major limitations. Herein, synthetic studies are presented to investigate the system's performance and improve its longevity for better use of this clean baseload power. The investigations are conducted by implementing appropriate equations of state to model state-of-the-art thermal and hydraulics processes in wellbores and considering various geometrical configurations to adopt proper design strategies. They provide insight for maximizing the generated thermal power, decreasing pumping energy, and avoiding production temperature drawdown. The results indicate that a stable thermal condition could be reached in which not only the temperature breakthrough is avoidable, but also the generated thermal power and production temperature continuously enhance over the project lifetime of one century. Analysis of the thermosiphon effect in the designed systems revealed that even with the pressure loss of 900 kPa at surface installations, the triggered natural flow rate is larger than 11 L/s. This thermosiphon flow rate yields the thermal power production of 2 MW and Cumulative extracted energy of 15 PJ over the project lifetime of 100 years. Restriction of this flow rate to 5 L/s leads to an average extraction temperature of 80 °C. It is also found that a change in the subsurface temperature gradient does not affect the optimal 2 km isolation length of the production well.

Improvement in the Performance of a Solar Hot Air Generator Using a Circular Cone

In this work the design and the study of a solar hot air generator, using concentrated solar radiation are proposed. This generator consists of a circular disc heated by the radiation reflected by a parabolic collector. The disc is placed at the entrance of an opened vertical cone. Under the thermal radiation effect emitted by the hot source, the internal wall of the generator is heated. The heating of the air at the generator-inlet generates a thermosiphon flow that is added to the one created by the hot source. In order to determine the critical parameters, the local convective transfer along the generator wall as well as the thermal and dynamic fields of the hot air at the cone exit for different angles and ratios of the rays are studied. It has been found out that the critical parameters of the conical generator, from which the thermal and dynamic characteristics of the hot air produced are R*=0.5; C=0.65; α=8°, have significantly improved. A comparative study of the hot air characteristics at the exit of the optimal conical generator, with two cylindrical and rectangular generators of the same geometrical characteristics is realized. The obtained results reveal relative increases of the mass flow rate of 78% and the heat flux absorbed by the air of 116% at the exit of the optimal cone compared to the cylinder. The results show also relative increases of the air temperature and the thermal flux absorbed by the air respectively of 400% and 129% at the exit of the optimal cone compared to the channel. A correlation between the hot source temperature and the air temperature at the exit of the optimal conical generator has been determined.

Impact of thermosiphon on nocturnal yield of conventional solar still

A mathematical model to predict the thermosiphon flow rate of a single slope solar still coupled with evacuated tube collector and its validation with experiments is presented in this paper. The objective of this experimental investigation is to determine the significance of thermosiphon flow rate and its impact on the fresh water yield during nocturnal conditions. The coupled system operates under thermosiphon through a heat exchanger placed within the basin water. The work was extended to study the influence of ambient temperature on thermosiphon head generated in the system. It was interesting to observe that higher temperature of water in Evacuated Tubular Collector (ETC) tank did not contribute to better flow rate at the inlet of the siphon circuit for varying exit temperatures. Numerical analysis was performed to determine the thermosiphon flow rate and it has good agreement with the experimental results.

Numerical investigation of a thermosiphon flow in a cylindrical enclosure with non-uniform boundary conditions

This research investigates the natural convection heat transfer phenomenon (“thermosiphon”) in a cylindrical enclosure with non-uniform boundary conditions, i.e., there is a peripheral temperature distribution along the inner hot cylinder while the outer cold cylinder remains at constant temperature. The analysis was performed using Finite Element Analysis method (FEA). The flow field parameters (temperature, velocity) and the overall heat transfer were analyzed and compared to the case of uniform boundary conditions. A correction factor has been proposed in order to use the classical empirical correlation for uniform boundary conditions with non-uniform boundary conditions. The primary insight of this research is that non-uniform boundary conditions enhance the heat transfer in the enclosure by up to 7%, depending on the non-uniformity of the hot wall temperature.

How to sustain a CO2-thermosiphon in a partially saturated geothermal reservoir: Lessons learned from field experiment and numerical modeling

CO2 has been proposed as a working fluid for geothermal energy production because of its ability to establish a self-sustaining CO2 thermosiphon, taking advantage of the strong temperature dependence of CO2 density. To test the concept of CO2 heat extraction, in January 2015 a CO2 thermosiphon was operated at the SECARB Cranfield Site, Cranfield, Mississippi, where a brine-saturated sand at a depth of 3.2km has been under near continuous CO2 flood since December 2009 as part of a U.S. Department of Energy demonstration of CO2 sequestration, resulting in a partially saturated reservoir surrounding a well pair. The lateral distance between the producer and injector was 112m at reservoir depth, a distance considered pre-commercial in scale, but great enough that thermal breakthrough was still not significant after several years of injection. Instead of producing power with a turbine, heat was extracted heat from recirculated fluid using a heat exchanger and portable chiller. The well field and surface equipment were instrumented to compare field observations with predicted responses from numerical models. Thermosiphon flow could be initiated by venting, but thereafter flow rate steadily declined, indicating that the thermosiphon was not sustainable. To model the system, the capability of T2Well, a fully coupled wellbore/reservoir numerical simulator, was expanded to enable simulation of the entire loop of fluid circulation in the fully-coupled system consisting of the injection/production wells, the reservoir, and the surface devices (heat exchanger, flow-rate regulator etc.). Combined with the newly developed TOUGH2 equation of state module called EOS7CMA, the enhanced T2Well was used prior to the field experiment to simulate the circulation of a CO2-H2O-CH4 mixture in a model geothermal system patterned after the Cranfield demonstration test. The model predicted that a sustainable thermosiphon could be achieved. After the field thermosiphon did not achieve the pre-test prediction of flow rates and thermosiphon sustainability, the numerical model was modified to improve realism and calibrate certain processes; it was then able to reproduce the major phenomena observed in the field. In a series of sensitivity studies, many factors were found that could potentially contribute to the failing of a sustainable thermosiphon. These factors could be categorized as two types: factors that increase the resistance to flow and factors that increase heat loss of the working fluid. The lessons learned can be applied to both future modeling and to achieving CO2-based geothermal reservoir exploitation.

Initial investigations of a combined photo-assisted water cleaner and thermal collector

Efficient solar thermal collectors incorporate thermosiphon effect to rid the costs associated with force flow systems, yet they lack the functions of solar photocatalytic collectors for water cleaning. The combination of hot and clean water from one collector is demonstrated in the present study by utilizing two chambers under Mirotherm® solar selective absorber. Top glazing has a transmittance of 71%. One chamber was closed and transports captured heat to the mixture chamber, which is open to a cold reservoir in a loop method. MB dye in water and AEROXIDE TiO2 P90 were utilized as the reagents. It has been observed that 1.2 ppm of MB dye can be cleaned using 127.4 mg*L-1 suspended UV-activated AEROXIDE TiO2 P90 that exhibits no sedimentation in the collector with thermosiphon flow. Thermal analysis was performed using ten thermocouples within the collector and non-contact temperature sensors above. The analysis of water heating thermal collector was studied for two modes; one with a controlled cold inlet and other with rising temperature inlet. We recommend that a controlled inlet temperature of 21.5 °C be utilized to acquire 40.1 °C increase in temperature, at which point the thermal efficiency of the collector is 67%.

On the Effect of Aspect Ratio of Open Heated Channel Including an Active Obstacle upon the Turbulent Characteristics of a Thermal Plume: Experimental Analysis

This paper reports an experimental investigation of aspect ratio effect of open vertical channel on turbulent characteristics of a thermal plume. The physical configuration is constituted essentially by a prallelepipedic channel and an obstacle of a rectangular section. The thermal plume is generated by a rectangular obstacle heated uniformly at the upper surface. This active source is placed at the entry of a vertical channel open at the ends. The symmetrical heating of channel walls by joule effect and by thermal radiation emitted by the plume, causes the appearance of a thermosiphon flow which interacts with this one. To investigate the flow fluctuations, an anemometer at constant current (CCA) is used. The results carried out with air (Pr = 0.71) are performed for Rayleigh number equal to 0.63 107 over a wide range of aspect ratio 1.25 ≤ A ≤ 30. Effects of this pertinent parameter are displayed upon thermal and dynamic turbulent fields. Using Taylor hypothesis, time and length scales of turbulent thermal field are studied. It is found that the turbulent characteristics of the flow are significantly influenced by the variation of aspect ratio. An optimum aspect ratio of channel characterized by a strong homogenization of turbulence of the flow is observed. The fine analysis of temperature fluctuations spectra shows the evolution of the vortices in energy cascade owing to the strong effect of thermosiphon flow which favours the vortex stretching.

Performance of a 10-kJ SMES model cooled by liquid hydrogen thermo-siphon flow for ASPCS study

We propose a new electrical power storage and stabilization system, called an Advanced Superconducting Power Conditioning System (ASPCS), which consists of superconducting magnetic energy storage (SMES) and hydrogen energy storage, converged on a liquid hydrogen station for fuel cell vehicles. A small 10- kJ SMES system, in which a BSCCO coil cooled by liquid hydrogen was installed, was developed to create an experimental model of an ASPCS. The SMES coil is conductively cooled by liquid hydrogen flow through a thermo-siphon line under a liquid hydrogen buffer tank. After fabrication of the system, cooldown tests were carried out using liquid hydrogen. The SMES coil was successfully charged up to a nominal current of 200 A. An eddy current loss, which was mainly induced in pure aluminum plates pasted onto each pancake coils for conduction cooling, was also measured.

Experimental study of the structure of a thermal plume inside a rectangular tunnel

The objective of this work is to experimentally simulate a plume developing inside a horizontal tunnel. The experimental device used in this simulation is essentially constituted of a hot disk, a rectangular tunnel and a ventilation system. The hot disk is heated by Joule effect to a constant and uniform temperature, and placed inside the tunnel. The hot source generates a thermal plume. We first studied the evolution of the thermal plume without ventilation system. The study of the average and fluctuating thermal and dynamic fields shows three zones during the vertical evolution of the free plume. A first zone close to the source, serving to the plume supply in fresh air, is characterized by the apparition of three escapes of the thermal plume. Followed by a second zone where the main escape undergoes a contraction. Finally, a third zone where the thermal plume accumulates and undergoes a flow upstream named backlayering and a flow downstream that borders the ceiling to leave by the free part of the tunnel. Keywords Fire plume, Fire tunnel, Thermal plume, turbulent natural convection. I. INTRODUCTION The thermal plume model is an experimental methodology usable easily to simulate the fire plume (1, 2). The thermal model contributes to better understand many practical fire problems such as problems associated with fire tunnel and flow encountered in fires of structural elements of buildings. The interaction of these material surroundings with the fire plumes reveals very complex physical mechanisms. From a fundamental viewpoint, the study of the interaction of the thermal plume with the vertical walls that surrounds it began with Agator's work (3) on the influence of a wall placed in the vicinity of the plume source. He noted that the plume is attracted toward the wall. A. O. M. Mahmoud and al. (4, 5, 6) are the first who were studied the evolution of a thermal plume in semi-confined geometry. They studied the evolution of a thermal plume produced by a flat disc heated at 300° C and placed at the entrance of an open-ended vertical cylinder. They noted that the plume interacts narrowly with the thermosiphon flow which develops along the internal wall of the cylinder. Contrary to previous works (7, 8, 9, 10), they noticed the appearance of a supplementary zone in addition to the two classic zones which characterize the vertical evolution of the free plume. Just above of the source, the instability zone is characterized by the formation of rotating rolls and by the existence of three extrema of temperature and velocity profiles. Higher, a second zone of turbulence pre- established followed by a last zone where the turbulence is fully established. J. Zinoubi et al. (11, 12) continued this experimental work by studying the form factors effect of the plume evolution inside a vertical cylinder. Using the visualization and analysis of the thermal and dynamic profiles of the flow, they showed the existence of three zones described previously. By studying the influence of the cylinder height, J. Zinoubi et al. (13) noted a blocking of the ascending flow in the third zone due to the lateral expansion of the plume. They also showed that a choice of the cylinder height not exceeding the second zone of the flow let us avoid this blocking. In order to determine the geometry effect, N. Taoufik et al. (14) studied the evolution of a thermal plume generated by a flat disc inside an open-ended rectangular canal. They noted the existence of the three zones observed in the cylindrical geometry. Also, they noticed the contraction of the rotating rolls size located in the first zone of the flow. Recently, A. O. Mahmoud et al. (1) studied the effects of source air entrainment on the flow structure induced by two heat sources, one placed at ground level, the other at a height above the ground. The experimental results permitted to specify that the additional vertical contribution of the air entrainment especially entails a substantial change of the flow structure of the plume, an important elongation of the height of the plume spread, a considerable increase of the flow rate of the plume and an important elevation of the thermal flux absorbed by the air. It is clear that these works were essentially interested to the determination of the effects of the emplacement, the heat release of the fire and the tunnel geometry on the critical ventilation velocity. The physical structure of the plume inside tunnel has not been studied. A fire plume inside tunnel has very complex

Temperature Spectra from a Turbulent Free Thermal Plume and in Interaction with its Material Environment

In this work we study the interaction of an axysymmetric thermal plume with a thermosiphon flow that surrounds it. The thermal plume is created by a circular disk heated by joule effect at constant temperature. The disk is placed at an open ended vertical cylinder on a quiet constant temperature. The internal wall of the cylinder heats up under the effect of thermal radiation emitted by the hot source. The confinement of the fluid causes, in the bottom part of the cylinder, an aspiration of the fresh air. It is a thermosiphon flow that comes to interact with the plume. By studying the average and fluctuating thermal fields it was found that the flow of the plume is strongly influenced by the presence of nearby walls. It was noted that the vertical transport becomes more intense and the structure of the flow becomes more turbulent. On the other hand, we attend a fast homogenization of the flow in the upper cylinder. To obtain more detailed information of this flow, we develops, during this study, a spectral analysis of the fluctuating thermal fields for the case of a plume evolving in unlimited and in an enclosed environment. The energy spectra study shows an important shift of the energy peaks toward the high frequencies under the effect of the thermosiphon. As destroying structures them on a big scale generated by the plume, the thermosiphon provokes a fast mixture of the fluid thus while leading to vortex of weaker size.

A palmtop PCR system with a disposable polymer chip operated by the thermosiphon effect

A small thermocyling system to perform DNA amplification by polymerase chain reaction (PCR) is presented in this report. PCR reactants are convected along a triangular closed-loop channel in a polymer chip by the thermosiphon effect. In an effort to develop a convection-based PCR for real application, we adopted a molded channel to define the flow path inside the chip, so that the chip may be suitable for disposability together with the merits of LOC; mass production, versatile integration and facile handling. We developed the geometry of the flow loop that made it easier to load the PCR reactants without air pockets inside. Based on systematic simulations and theoretical considerations of buoyant flows, the loop channel was designed to acquire an optimized flow for PCR. A PCR sample was dropped on a chip to fill the loop channel, and the chip was inserted into a slot of a heating block unit that was composed of three metal blocks with different temperatures. The temperature differences within the closed loop gave rise to buoyancy differences and the liquid reactant continuously circulated along the closed loop by the thermosiphon effect. Because there was no loss of time among the temperature shifts in the reaction steps, approximately 10 min were sufficient for the detectable amplification of 127 bp target gene from 10 pg microl(-1) of PCR fragments. In addition, it took 20 min for the mass amplification of 470 bp gene from PCR fragments or genomic DNA. The entire PCR system is compact enough even to be palmtop because it requires only a tiny temperature controller for a self-actuated thermosiphon flow. This thermocycling system would be a prototypical model for the field application of PCR.

Heat transfer characteristics of two-phase He I (4.2 K) thermosiphon flow

In the framework of the cryogenic cooling system design of a large superconducting magnet under construction at CERN-Geneva, heat transfer in two-phase He I natural circulation loop has been investigated experimentally. The experiments were conducted on a 2 m thermosiphon loop with copper tube of 10 mm inner diameter uniformly heated over a length of 0.95 m. All data were obtained near atmospheric pressure. Evolution of the exit vapour quality and wall superheat as a function of heat flux are presented and analyzed. A comparison between the two-phase heat transfer coefficient h TP determined in our study and the most relevant correlations available in literature is made. Further, we predict h TP with a correlation based on the combining effects of forced convection and nucleate boiling by a power-type asymptotic model. Finally, we present the boiling crisis study and we propose a critical heat flux correlation as a function of channel height to diameter ratio ( z/ D) to model our experimental results.

Study of hot air generator with quasi-uniform temperature using concentrated solar radiation: Influence of the shape parameters

The non-uniformity of the air temperatures and the slow flow rate at the plane collector exit constitute the main cause of the limitations of the solar drying systems. In order to obtain an uniform and a variable flow rate for different uses, a hot air generator using concentrated solar radiation is proposed. To improve the thermal efficiency of the generator, a study of the influence of different shape parameters is realized. The generator is simulated in the laboratory while investigating the flow induced by a circular disc heated uniformly by Joule effect at constant temperature. This disc is placed at the entrance of an open ended vertical cylinder of a larger diameter. Thermal radiation emitted by the hot disc heats the cylinder wall. The heating of the fluid at the cylinder-inlet generates a thermosiphon flow around the one created by the hot disc. The comparison of the velocity and the temperature profiles of the resulting flow permits to determine the influence of the cylinder height, the vertical source-cylinder spacing and the radius ratio, on the resulting flow at the system exit. Thus, a judicious choice of the shape parameters entails an improvement of the flow rate as well as the thermal flux absorbed by the air and a good homogenization of the air temperature at the generator exit.

Influence of the vertical source–cylinder spacing on the interaction of a thermal plume with a thermosiphon flow: an experimental study

The present work reports an experimental study of a turbulent natural convection flow resulting from the interaction of an axisymmetric thermal plume with a thermosiphon flow. A circular disk, heated by Joule effect at constant temperature (300 °C), generates the plume. This disk is placed at the entrance of an open-ended insulated vertical cylinder of a larger diameter. Thermal radiation emitted by the hot disk heats the cylinder wall. The pressure reduction due to the acceleration of the flow at the cylinder-inlet generates a thermosiphon around the thermal plume which modifies its global structure considerably. In this paper we expose results of the experimental study related to the heating source position. We study particularly the effects of the source–cylinder spacing on the flow structure. A comparative study is presented for different configurations. This study is based on experimental results concerning the average and fluctuating fields. These results are analysed and discussed in order to understand more deeply the flow development mechanisms generated by buoyancy forces. The use of thermal and dynamic factors of flatness and skewness allow a better knowledge of the flow fine structure.

Solar cooking system with or without heat storage for families and institutions

A solar cooking system, with or without temporary heat storage, has been developed and installed in different countries of the world. The two basic system components are the solar collectors with reflectors and a cooking unit. When thermal storage is needed, a tank filled with pebbles is added to the system. The working fluid, usually a vegetable oil, circulates in natural, thermo-siphon flow through a copper piping that connects the components. The system presents interesting features such as the possibility of indoor and night cooking, heat flow control in the pots, modularity, and the possibility of further adjustment to incorporate a baking oven. The first prototype of this collector–cooker system was built and installed in the city of Juelich, Germany. Further development and adjustment were performed for the next versions, which were brought to India and Mali. Other systems were installed in Africa (Burkina Faso and South Africa) and in South and Central America. About 250 systems have been built in different sizes to be used by single families or by government institutions. The excellent experimental results are presented in this article.

Performance of an unglazed textile-plastic solar absorber

A simple solar water heating system comprising an unglazed textile-plastic absorber and an uninsulated plastic accumulator tank with thermosiphon flow of heat transporting fluid has been tested under outdoor conditions. The unglazed textile-plastic absorber is very sensitive to changes in climatic conditions. The overall efficiency and the response time to changes in irradiation depend also from ambient air temperature, absorber temperature and intensity of solar radiation. Under good conditions its efficiency is comparable with that of sophisticated metallic collectors (e.g. with selective absorbing layers), but on cold days when the intensity of solar radiation is low it is considerably lower. For some applications, where only a small rise in water temperature is needed, such as swimming pool water heating or water preheating for irrigation in greenhouses, they may be more cost effective than metallic solar collectors, when their low price is taken into account.

Experimental observations on a continuous flow type domestic solar water heater

The operation of a continuous flow type domestic solar water heater is explained with the help of a schematic diagram. Steady state thermal analysis of such a water heater is carried out. A water heater of this type with a collector area of 1 m 2 is subjected to tests. The experimental results are compared with predicted results. The experimental results have also been compared with those obtained from a water heater operating on thermosiphon flow. The performance of the continuous flow type water heater is shown to be comparable to that of the thermosiphon flow type. It is concluded that, in the Indian context, the continuous flow type water heater has more advantages than the thermosiphon flow type water heater in view of its low capital cost and the ease of operation and maintenance.

Domestic solar water heater—continuous flow type

Problems in domestic solar water heaters, operating on thermosiphon flow, are discussed. A continuous flow type water heater is suggested which will overcome the problems in the thermosiphon flow water heater. Thermal analysis of such a system is carried out for winter and summer performances. The performance of the continuous flow type water heater is comparable to that of the thermosiphon flow type. It is concluded that, in the Indian context, the continuous flow type water heater has more advantages than the thermosiphon flow type water heater. The initial cost of the former is less than that of the latter type. Also, the operation and maintenance of the continuous flow type is simple.