Guidelines for Circular Heat Pipe Thermal Testing

For today’s blog I’d like to present some work several of my colleagues and I have been working on for submittal to the fall session of the JEDEC Committee: Guidelines for Heat Pipe Thermal Test Set for Testing of Circular Heat Pipes.

As we’re always open to comments before submittal, please feel free to do so at the bottom of this blog or in one of the LinkedIn groups in which this is posted.

1.  Scope

This document provides guidelines to establish the functionality required of a circular heat pipe test set which can be used to consistently measure the performance of a straight, circular heat pipe. This document, coupled with JESD51-12-01 which defines the heat pipe thermal test procedure, JESD51-73 which defines terms and definitions and JESD51-xx which defines the test results reporting format, provide a family of guidance which allow for consistency of process.

The primary goal of this document is to establish consistent test set parameters for heat input, heat removal and power and temperature measurement such that presented heat pipe test results are consistent across the industry. Suppliers may choose to limit the extent of data extracted from a test but for that data which is presented, the results should conform to the test set criteria established by this document. In doing so, the output of the thermal test set defined by this document provides a common basis for discussion between heat pipe suppliers and users.

2.  Normative References

The following standards contain provisions that, through reference in this text, constitute provisions of this guideline. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on these standards are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below.

[1] JESD51-70, Overview

[2] JESD51-73, Terms and Definitions

[3] JESD51-12-01, Circular Heat Pipe Test Procedure

[4] JESD51-xx, Heat Pipe Thermal Test Data Sheet

3.  Terms, Definitions, Symbols and Abbreviations

Standard JESD51-70 [1] is an overview document addressing the performance testing and characterization of heat pipes (circular, flat and vapor chamber). Terms, definitions and symbols are defined in JESD51-73 [2].

4.  Thermal test set variables

The following are a list of the process variable outputs which the referenced thermal test set must be capable of controlling/outputting:

  •  Condenser temperature, Tc (°C)
  • Evaporator temperature, Te (°C)
  • Adiabatic temperature, Ta-e, Ta-c (°C)
  • Input power (W)
  • Pipe orientation with respect to gravity

5.  Components of the Thermal Test Set which Impact Consistency of Test Results

To allow for consistency and comparability of thermal test results, factors which impact the measured performance of a heat pipe must be kept consistent.

The items defined in [4] above are the test outputs required to effectively compare the thermal performance capabilities of a heat pipe. To facilitate consistency and comparability in generating those results, the test set elements must be consistent so that the results from the test set are comparable from test to test, be they multiple tests from a single vendor or comparative results from alternate vendors. The critical elements of the test set are identified below and the recommended configuration of this standard for each of these elements will be fully defined in subsequent sections of this standard.

  • Evaporator – the heat input length and the definition of single sided or fully encased, uniformity of clamping pressure, uniformity of heat flux input to the heat pipe, the interface between the evaporator section of the heat pipe and the heat source
  • Condenser – the heat rejection length and the definition of single sided or fully encased, the uniformity and rate of heat rejection from the condenser attachment, the interface between the condenser section of the heat pipe and the heat sink, ability to control/maintain heat pipe condenser temperature as power level is adjusted
  • Temperature measurement – temperature measurement locations and temperature measurement method
  • Power input – ability to incrementally adjust power level
  • Orientation with respect to gravity – ability to maintain/adjust orientation of the test article with respect to gravity

6.  Thermal Test Set Description

A representative configuration of the circular heat pipe test fixture is shown in Figure 1.

Circular Heat Pipe Test Set

Figure 1 – Circular Heat Pipe Test Set

 

The evaporator block of the test set should fully encapsulate the heated length of the heat pipe. It should be split along its centerline and should be machined to assure that it exerts a uniform clamping force on the heat pipe such that when the evaporator block is clamped in place the heat pipe cannot be rotated or extracted by pulling on the pipe with manual force.

The evaporator block should be heated on only the lower clamping surface. The evaporator block material should be copper. The conduction length between the nearest heat pipe surface and the electrical heater surface should be a minimum of two heat pipe diameters separated. (See Figure 2)

The evaporator block should have a passage for a spring loaded thermocouple that will interface on the heat pipe surface, on the heated side, centered in the heated length.

The clamping force, when the heat pipe is inserted into the evaporator block should be a minimum of xxx N/m2 of force.

The interface between the heat pipe and the evaporator block should be coated with a light layer of thermal interface grease prior to installation of the test article.

Figure 2 - Evaporator Block Design

Figure 2 – Evaporator Block Design

 
The condenser block of the test set should fully encapsulate the cooled length of the heat pipe. It should be split along its centerline and should be machined to assure that it exerts a uniform clamping force on the heat pipe such that when the condenser block is clamped in place the heat pipe cannot be rotated or extracted by pulling on the pipe with manual force.

The condenser block should be cooled on only the lower clamping surface. The condenser block material should be copper. The conduction length between the nearest heat pipe surface and the liquid coolant passage in the condenser block should be a minimum of two heat pipe diameters separated. The method of cooling the condenser block should be liquid cooling circulated though coolant passages in the condenser block. (See Figure 3)

The condenser block should have a passage for a spring loaded thermocouple that will interface on the heat pipe surface, on the coolant side, centered in the heated length.

The clamping force, when the heat pipe is inserted into the condenser block should be a minimum of xxx N/m2 of force.

The interface between the heat pipe and the condenser block should be coated with a light layer of thermal interface grease prior to installation of the test article.

Figure 3 - Condenser Block Design

Figure 3 – Condenser Block Design

 

The evaporator and condenser blocks should be located on a ridged base (noted as adjustable length base in Figure 1) that keeps the heat pipe, when clamped in the evaporator block and condenser block oriented in the same plane in both the X and Y axis.

The rigid base should be designed such that it can be rotated, allowing the heat pipe test orientation to be changed from horizontal to gravity aided (evaporator down) or gravity inhibited (evaporator up).

7.  Thermocouple Measurements/Locations

The following temperatures shall be measured as outputs from the test set.

  • Theater
  • Tevaporator
  • Tadiabatic-evaporator
  • Tadiabatic-condenser
  • Tcondenser
  • Theat sink

The location of these temperature measurements is identified in Figure 4.

In the testing of the heat pipe, this data can be recorded manually or as an element in a data acquisition system.

Figure 4 - Temperature Measurement Locations

Figure 4 – Temperature Measurement Locations

 

8.  Power Input

Power input to the heat pipe is delivered via electric resistance heaters. The electrical power supplied to those resistance heaters should be delivered via a regulated power supply with power measurement capability. This can be a watt-meter or measurement of voltage/amperage draw. This input power will be recorded as the test power.

This data can be recorded manually or as an element in a data acquisition system.

9.  Test Set Orientation

A method of determining the heat pipe orientation should be included as a part of the test set.

This data can be recorded manually or as an element in a data acquisition system.

10.  Power Input

Power input to the heat pipe is delivered via electric resistance heaters. The electrical power supplied to those resistance heaters should be delivered via a regulated power supply with power measurement capability. This can be a watt-meter or measurement of voltage/amperage draw. This input power will be recorded as the test power.

This data can be recorded manually or as an element in a data acquisition system.


Contributors to this document include:

George Meyer – Celsia Inc.

Jim Petroski – Mentor Graphics

Bernie Siegal – TEA

Jerry Toth – Thermacore