Periodic Reporting for period 1 - POTPLOS (Performance optimization of two-phase passive loop system)
Reporting period: 2020-10-01 to 2022-09-30
Innovative heat transport devices are needed to break-though the current conflicting situation of electronic devices for miniaturizing and increased heat generation. A high operating temperature reduces the reliability and performance of electronics, and eventually causes fatal failure. Thus, it is essential to spread or remove the heat from heat-generating areas to prevent hot spot. Cooling the heat-generating electronics directly by air or liquid can be the simplest and most effective way to remove heat, but it requires particular layouts that are sometimes not feasible. In such cases, heat transfer devices that efficiently transport heat from the heat-generating part to the cooled part are useful. Among these, two-phase flow heat transport devices, known as heat pipes, are very effective devices because they can transport heat over long distances with extremely small temperature drops.
A pulsating heat pipe (PHP) is one of the latest “evolved” heat pipes. It consists of a capillary tube meandering between an evaporator (heated section) and a condenser (cooled section), as shown in Fig.1. The PHP is partially filled with a working fluid that exists as a mixture of liquid slugs and vapor plugs. Heat is transferred from the evaporator to the condenser by the self-excited oscillation of vapor and liquid. PHPs have many advantages, including having a simple construction, being lightweight and flexible. Nevertheless, PHPs have not yet been put to practical use.
To boost the practical use of PHPs in different industrial applications, it is essential to establish a predictive model of PHP heat transfer performance, particularly at the operating limit. The operating limit of a PHP is the state where the working fluid inside the PHP does not undergo self-oscillation and the heat transfer from the evaporator to the condenser stops. If the heat input to the evaporator continues after the operating limit is reached, the evaporator temperature rises immediately, and the internal pressure of the PHP also increases. In the worst case, this destroys the PHP. In addition, the equipment cooled by the PHP heats up due to the lack of heat transfer, which may cause serious damage to the equipment. Therefore, it is essential to design PHPs, so they do not reach their operating limit during operation.
The overall objective of this project is to lead to better understanding of the fundamental physical mechanisms governing the behavior of PHPs, which are, so far, only partially understood and to provide an optimal design solution that maximizes the heat transport capability of PHPs using a predictive model.
The innovative approach applied in the project, the combination of advanced measurement with a high-resolution, high-speed infrared(IR) camera and inverse heat conduction problem (IHCP) technique worked effectively to investigate the local thermal phenomena related to liquid-vapor interactions and estimate the local heat fluxes exchanged between the fluid: The study of single-loop and multi-turn PHPs revealed that single-loop PHP reaches the operating limit in the different mechanisms from the multi-turn ones. In addition, there are three different mechanisms of the operating limit of the multi-turn PHP depending on the filling ratios: low (below 30%), high (above 80%), and medium (between low and high). The performance was optimized when the PHP has the medium filling ratio as the PHP can operates with the lowest thermal resistance until the evaporator temperature reaches to the critical temperature of the working fluid.
A pulsating heat pipe (PHP) is one of the latest “evolved” heat pipes. It consists of a capillary tube meandering between an evaporator (heated section) and a condenser (cooled section), as shown in Fig.1. The PHP is partially filled with a working fluid that exists as a mixture of liquid slugs and vapor plugs. Heat is transferred from the evaporator to the condenser by the self-excited oscillation of vapor and liquid. PHPs have many advantages, including having a simple construction, being lightweight and flexible. Nevertheless, PHPs have not yet been put to practical use.
To boost the practical use of PHPs in different industrial applications, it is essential to establish a predictive model of PHP heat transfer performance, particularly at the operating limit. The operating limit of a PHP is the state where the working fluid inside the PHP does not undergo self-oscillation and the heat transfer from the evaporator to the condenser stops. If the heat input to the evaporator continues after the operating limit is reached, the evaporator temperature rises immediately, and the internal pressure of the PHP also increases. In the worst case, this destroys the PHP. In addition, the equipment cooled by the PHP heats up due to the lack of heat transfer, which may cause serious damage to the equipment. Therefore, it is essential to design PHPs, so they do not reach their operating limit during operation.
The overall objective of this project is to lead to better understanding of the fundamental physical mechanisms governing the behavior of PHPs, which are, so far, only partially understood and to provide an optimal design solution that maximizes the heat transport capability of PHPs using a predictive model.
The innovative approach applied in the project, the combination of advanced measurement with a high-resolution, high-speed infrared(IR) camera and inverse heat conduction problem (IHCP) technique worked effectively to investigate the local thermal phenomena related to liquid-vapor interactions and estimate the local heat fluxes exchanged between the fluid: The study of single-loop and multi-turn PHPs revealed that single-loop PHP reaches the operating limit in the different mechanisms from the multi-turn ones. In addition, there are three different mechanisms of the operating limit of the multi-turn PHP depending on the filling ratios: low (below 30%), high (above 80%), and medium (between low and high). The performance was optimized when the PHP has the medium filling ratio as the PHP can operates with the lowest thermal resistance until the evaporator temperature reaches to the critical temperature of the working fluid.
Work was conducted via 6 work packages (WPs). WP1 was experimental studies with a single loop. The Researcher designed and manufactured single-loop PHPs with two different diameters and tested with two different working fluids, ethanol and R134a. An IR camera was placed facing to the PHP so that the image of the condenser was caught. The local heat flux was estimated by solving the IHCP with the IR image as an input data. It was revealed that the PHP operated differently depending on the working fluid, tube diameters, and cooling methods. The results were presented in an international conference and will be reported in a forthcoming paper of a peer-reviewed scientific journal.
WP2 was experimental studies with multi-turn PHP. The Researcher designed and manufactured multi-turn PHPs with stainless-steel. The PHP was set in from of the low-speed tunnel to cool the condenser, with faced to the IR camera. There are three different mechanisms of the operating limit of the multi-turn PHP depending on the filling ratios: low (below 30%), high (above 80%), and medium (between low and high). The performance was optimized when the PHP has the medium filling ratio as the PHP can operates with the lowest thermal resistance until the evaporator temperature reaches to the critical temperature of the working fluid. The results will be presented in an international conference in February 2023 and will be reported in a forthcoming paper of a peer-reviewed scientific journal.
WP3 was modelling. A robust, efficient algorithm for solving the IHCP was established and adopted to estimate the local heat flux for single-loop and multi-turn PHPs. This innovative approach and local heat flux estimation on a PHP was presented in International Heat Pipe Conference and the presented paper was received the best paper award in the fundamental research category (figure 2 shows an image of the celebration of it). The results were also reported in two peer-reviewed scientific journals.
In the training package WP4, The Researcher was trained for IHCP algorithms and modelling, IR measurement and data processing, and thermal tomography technology, along with conducting WP1 and WP3. The skills for dissemination and communication of research were trained though writing the papers, presenting in international conferences, and the dissemination activities listed below. Secondment at a leading SME (small-medium sized enterprise) in the two-phase heat transfer device fields was conducted in Belgium and the Researcher performed experiments of multi-turn. Figure 3 is an image taken there. The results were presented in an international conference and will be reported in a forthcoming paper of a peer-reviewed scientific journal.
The management of all scientific, contractual, financial, and legal aspects, including risk management was conducted under WP5.
WP6 was Communication, dissemination, and exploitation. Using multiple channels and networks, including university-owned ones such as the press office and official social media, a wide range of dissemination activities were effectively implemented to different targets, raising awareness of the project both nationally and internationally. The images of Figs. 4 and 5 were “Christmas-tree PHP” fabricated by the Researcher and supervisor and figure 6 was taken on European Researchers’ Night 2021.
WP2 was experimental studies with multi-turn PHP. The Researcher designed and manufactured multi-turn PHPs with stainless-steel. The PHP was set in from of the low-speed tunnel to cool the condenser, with faced to the IR camera. There are three different mechanisms of the operating limit of the multi-turn PHP depending on the filling ratios: low (below 30%), high (above 80%), and medium (between low and high). The performance was optimized when the PHP has the medium filling ratio as the PHP can operates with the lowest thermal resistance until the evaporator temperature reaches to the critical temperature of the working fluid. The results will be presented in an international conference in February 2023 and will be reported in a forthcoming paper of a peer-reviewed scientific journal.
WP3 was modelling. A robust, efficient algorithm for solving the IHCP was established and adopted to estimate the local heat flux for single-loop and multi-turn PHPs. This innovative approach and local heat flux estimation on a PHP was presented in International Heat Pipe Conference and the presented paper was received the best paper award in the fundamental research category (figure 2 shows an image of the celebration of it). The results were also reported in two peer-reviewed scientific journals.
In the training package WP4, The Researcher was trained for IHCP algorithms and modelling, IR measurement and data processing, and thermal tomography technology, along with conducting WP1 and WP3. The skills for dissemination and communication of research were trained though writing the papers, presenting in international conferences, and the dissemination activities listed below. Secondment at a leading SME (small-medium sized enterprise) in the two-phase heat transfer device fields was conducted in Belgium and the Researcher performed experiments of multi-turn. Figure 3 is an image taken there. The results were presented in an international conference and will be reported in a forthcoming paper of a peer-reviewed scientific journal.
The management of all scientific, contractual, financial, and legal aspects, including risk management was conducted under WP5.
WP6 was Communication, dissemination, and exploitation. Using multiple channels and networks, including university-owned ones such as the press office and official social media, a wide range of dissemination activities were effectively implemented to different targets, raising awareness of the project both nationally and internationally. The images of Figs. 4 and 5 were “Christmas-tree PHP” fabricated by the Researcher and supervisor and figure 6 was taken on European Researchers’ Night 2021.
The project result could lead to socio-economic benefits in various sectors through a chain optimization enabled by the diffusion of technologies for novel process intensification. For example, innovative, more efficient heat exchangers in thermal treatment plants and in devices for cooling electronics could substantially improve their overall energy efficiency, and thus reduce energy consumption and CO2 emissions, resulting in a beneficial effect on the environment.