Experimental Cooling Performance Evaluation of Different Coolants for Data Centre

The direct liquid cooling (DLC) of the data center is becoming popular due to its higher heat removal from the computer chips. The direct liquid cooling method is more effective than the conventional air-cooling system and reduces the higher infrastructure and maintenance costs. The DLC reduces the chip failure rate drastically and increases the life of the data centers. Different liquids can be used as a coolant and some manufacturers are coming up with different coolants where the liquid has high thermal efficiency and is electrically non-conductive. In this article, a heat-transfer cold plate made of aluminum is designed and a different combination of heat-transfer liquids (Distilled water, Ethylene Glycol, and Polyethylene Glycol) is tested to find a comparatively better combination of heat-transfer liquid. It was observed that the combination of Ethylene Glycol and distilled water performs better than other combinations. It was also found that the coolant flow rate plays an important role in the cooling of the chips as well.


Introduction
The data center is a house of hardware and software infrastructure that supports IT (Information Technologies) businesses, government agencies, banks, cloud computing, cloud service, and many others.Data centers are massive computer warehouses that store large volumes of data for different purposes to provide different services.The data center industry experienced a remarkable expansion in recent years as a result of rising demand for data processing and advances in computer and electronic technology [1].The temperature and humidity of the data center environment are controlled to ensure the proper operation of the equipment.The proper operation of the data center requires centralized airconditioned facilities and an uninterrupted power supply is needed.The centralized air-conditioned system requires big investments to build the infrastructure and maintain and if the power fails the data center is in immediate danger of failing as the required temporary backup power system (Stationary power generator) capacity is also high.The growing number of data centers all around the world will undoubtedly increase energy consumption and energy costs.The data center consumes almost 1.8% of total energy consumed in the USA and that is in the year 2014 alone [1].In the data center, the cooling system consumes almost 38% of the total power consumption of the data server [2].The heat-generating chips like central processing unit (CPU), graphics processing unit (GPU), microcontrollers, and integrated circuits need IOP Publishing doi:10.1088/1757-899X/1305/1/012018 2 to cool down.The heat generated from these chips is taken away through the fan and fin mechanism or heat pipe and fin mechanism.That mechanism releases heat inside the server room and the server room needs to cool down through central air conditioning.The humidity of the server room needs to be controlled as well.There is a possibility of condensing the air moisture on the fan coil unit if the humidity is higher inside the data center.The continuous demand for data storage will increase with time and the number of data centers will increase all around the world as well [3].The DLC can replace the data center air conditioning system hence reducing power consumption [4,5].There are two different ways the DLC can be applied.There is a single liquid/coolant system where the cold plates are directly mounted on the chip and the liquid flows through the cold plate.The heat from the computer chip is transferred to the cold plate and the cold liquid takes away the heat from the cold plate.The heated liquid leaves the cold plate and goes through some heat exchanger and eventually goes back to the cold plate to complete the cycle.There is a dual liquid/coolant system where two different liquids are used.In the primary fluid network, one liquid goes through the cold plate and removes the heat from the chips.The secondary fluid network removes the heat from the first fluid through a heat exchanger.The liquid is moved through the fluid networks via pumps, valves, tubes, and different connectors.The connectors are designed especially for the dripless operation of the fluid networks.There are different liquids/coolants commercially available that can be purchased and used for this application.In this study, conventional and comparatively cheaper heat transfer liquid such as distilled water, ethylene glycol, and polyethylene glycol-based coolant is used to investigate the effectiveness of these liquids in cooling the data center.It can be noted here that, ethylene glycol-based coolant has been used in the auto industry for quite some time and has proven to be excellent in heat transferring from auto engines [6,7].

Methodology
This study was focused on two main areas.First, a cold plate was designed, preparing a heat source, and building a single fluid network to cool down the cold plate.The cold plate was mounted on the chip in this case the equivalent heat source of a computer processor.The second part is preparing the liquid out of distilled water, ethylene glycol, and polyethylene glycol and using their different compositions to remove heat from the heat-designed cold plate.The cold plate is designed in such a way that the liquid inside the cold plate gets the maximum possible time and maximum surface area to transfer the heat from the chip.The design consists of a spiral fin inside the cold plate.The plate is made out of aluminum (6061) and machined with a VF2 CNC milling machine from HASS.The material was tested with x-ray diffraction from Olympus (Alloy+).A CFD simulation is performed to observe the flow pattern of the fluid to make sure the designed spiral is effective in transferring heat.The heater was made with nichrome wire to mimic the heated chip and allowed to raise the temperature of the cold plate to a maximum of 40° C and sometimes 60° C (during the investigation of the coolant flow rate effect).The casing temperature of an Intel processor having a 125-watt rating does not exceed more than 71.9°C [8].The casing temperature is muloweress for another lower-rated processor [8].In modern computers, the cooling of the processor starts as soon as the computer starts.The ambient temperature in Bangladesh sometimes exceeds more than 40°C but the average temperature in Bangladesh never exceeds more than 32.5°C [9].The properties of ethylene glycol (Merck) and polyethylene glycol are shown in Table 1.

Design of the Cold Plate
The cold plate design is very important and it should have some characteristics to avoid damage to the computer chips.The cold plate material should be corrosion resistant, have high heat conductivity, good manufacturability, small thermal expansion, and less oxidation.Commercial cold plates are usually made out of stainless steel.However, in this study aluminum material is used for test purposes.
The physical dimensions of the cold plate are shown in Table 2 and the SolidWorks design is shown in Figure 1.The objective of the spiral-designed fin inside the cold plate is to increase the inner surface area of the cold plate so that the liquid comes in contact with more surfaces for more heat transfer.The liquid/coolant will travel a longer path due to the spiral and will take more time to remove the heat from the cold plate.The CFD simulation became the most important methodology for analysing thermo-fluidic simulation scenarios for a data server [10].A CFD simulation in Ansys was run to visualize the flow nature of the fluid inside the cold plate.The flow simulation shows that the flow is turbulent and follows the flow path without much flow separation.An in-depth heat transfer analysis was possible to perform but that was not the objective of this study.Therefore, the mesh dependency test and experimental validation of the flow velocity were not performed.The meshing and velocity streamline image is shown in Figure 2. The velocity streamline shows turbulence in the flow which is needed for better heat transfer as the liquid induces self-mixing.Later, the cold plate is manufactured with aluminum material with VF2 from HASS as shown in Figure 3.

Experimental Setup
The experimental setup has two main components, a fluid flow network, and a monitoringmeasurement system.The fluid network has a fluid reservoir, a fluid pump coupled with a DC (12V) motor, a flow rate sensor (YF-S201, Measuring Range 1-30 L/min), a cold plate, a heater, and a radiator.The cold plate is attached to the heater with thermal paste (HY-510) to ensure the maximum possible heat transfer from the chip to the cold plate.
The monitoring and measurement system consists of three temperature measurements with K-type thermocouples.One thermocouple was sandwiched between the chip and cold plate to monitor the temperature and if the temperature increases to 40° C the signal is given to start the coolant pump.In some cases, the heater temperature was set to 60° C. The other two thermocouples monitored the inlet and outlet coolant temperature of the cold plate.Figure 4 shows the schematic of the experimental setup and Figure 5 shows the experimental setup.An Arduino-based control and measurement system was developed to automate the process.The thermocouple and flow sensor were calibrated with another reference sensor.The temperature measurement was accurate te ±0.07°C and the flow rate was ±7.6 ml/min.3.

Experimental Results and Discussion
The experimental results shown in Figure 6 illustrate that the heat removal rate from the cold pate is highest for the distilled water and 5-10% Ethylene glycol for the next.This could be due to the lower viscosity of water which increases more flow rate during the experiment.The dielectric potential was tested (LY JJ-II-80, Insulating Oil Breakdown Voltage Tester) for the distilled water and 5% of Ethylene Glycol but no significant difference was observed shown in Figure 7.If the CPU temperature was raised to 60 °C, the 5% Ethylene Glycol and 80 % Ethylene Glycol showed a similar trend as for 40 °C.However, the higher temperature of the CPU requires more time to reach the steady state temperature provided that all other parameters remain unchanged.The boiling temperature of the 5% Ethylene Glycol was also measured and the boiling temperature came out to be 99 °C.The boiling temperature of Ethylene glycol is around 197 °C and that came down to 99 °C for the 5% Ethylene Glycol which is still higher than the Intel 125-watt processor casing temperature [8].If the concentrations of Polyethylene Glycol in water are compared to Ethylene Glycol, then the resulting trend is similar to the Ethylene Glycol shown in Figure 9 even for higher CPU temperature shown in Figure 10.The lowest concentration (5%) of polyethylene glycol reaches the steady state region faster than the higher concentration of Polyethylene Glycol (80%).However, the lower concentration (5%) of Ethylene Glycol reaches steady state temperature faster than the lower concentration of Polyethylene Glycol shown in Figure 11.This indicates that the Ethylene Glycol solution has better heat removal capacity than the Polyethylene Glycol solution.The flow rate is also changed and we can see the effect of the coolant flow rate on heat removal rate.The higher flow rate also increases the power consumption on the pump side.The power consumption for flowing the liquid is also an important factor and research was conducted to develop a model to predict this power consumption on a small scale [11].If the viscosity of the coolant is increased then the power consumption will increase even more.The 5% Ethylene Glycol flow rate was increased IOP Publishing doi:10.1088/1757-899X/1305/1/01201810 from 9.75 ml/sec to 14.5 ml/sec.It was observed that the higher flow rate tends to reach the steady state temperature faster as time passes shown in Figure 12.The higher flow rate increases the fluid mass flow through the cold plate and takes more heat but at the same time reduces the time for heat transfer.On the other hand, at the radiator section, the higher mass flow rate reduces the heat transfer rate as well.Therefore, the coolant is soaking heat less in the cold plate and rejecting less heat as well for a higher flow rate.Again, the higher flow rate will increase the energy consumption at the pump side.Therefore, the optimum flow rate of the coolant may depend on several factors such as heat load, power consumption limit for the pump, heat exchanger efficiency, and cold plate design.

Figure 12.
The effect of flow rate on the cooling of cold plate at different flow rates.

Conclusion
After attempting the first experiments it was found out that distilled water gives the quickest way to steady state.However, the 5% ethylene glycol and 95% distilled water mixture gives the next highest rate of heat reduction from the cold plate.Ethylene glycol-based coolant has been used for a very long time in the automotive industry to cool engines as a solution that has lower freezing temperatures.The 95 % Ethylene Glycol mixed with 5% distilled water solution has a boiling point (99° C).The higher boiling point will not create vapour pressure inside the fluid circuit.In the second experiment, the temperature reduction rate is the highest for 14.5 ml/sec, and for 9.75 ml/sec, the flow rate of temperature reduction is the lowest.The temperature was controlled with this application without issues.
The polyethylene/Ethylene glycol solution is not electrically insulating material therefore it still poses a damage threat if there is a leakage issue.The market of DLC is increasing and gradually replacing the old high energy-consuming system.The current world is unpredictable when it comes to energy prices due to geopolitical conflicts and artificial market control.Therefore, having an energy-efficient system is a futuristic thinking for most companies.There is immersion technology as well but the amount of liquid required and handling them is much messier than the DLC system the immersion technology is also preferred by some data centers as it has the highest thermal efficiency.In this study, even the distilled water shows maximum heat transfer quality but it has many disadvantages, among which, corrosion and electrical conductivity are the main issues.However, it is the cheapest and most abandoned material and if some companies, do not have access to other chemicals, a leakproof design and rust-proof material choice will just do fine in warmer weather

Table 2 :6Figure 1 .
Figure 1.The designed cold plate (a) Top lid and (b) Bottom part

Figure 9 . 9 Figure 10 .Figure 11 :
Figure 9.The effect of Polyethylene Glycol percentage on the cooling of the cold plate

Table 1 .
Properties of Ethylene Glycol Used in the Experiment

Table 3 :
The combination of distilled water, Ethylene Glycol, and Polyethylene Glycol was tested in this study